Outbreaks of violence in Nigeria have killed at least 14,000 people and displaced hundreds of thousands since the country returned to democracy in 1999, ending 16 years of military rule.

Flawed and violent elections in April 2007 thwarted international hopes that Africa's most populous nation would set an example at the ballot box, and prove its potential as a political leader for the continent.

Around 65 people died in crime linked to polls for state governors on April 14 and presidential and national assembly elections a week later.

The ruling People's Democratic Party (PDP) claimed a landslide victory in both sets of elections. PDP presidential candidate Umaru Yar'Adua, a northern Muslim, won close to four times as many votes as his closest rival, former army strongman Muhammadu Buhari of the All Nigeria People's Party. Opposition groups rejected the swiftly announced results.

Yar'Adua was inaugurated president in May 2007, but his election victory has been contested for months.

Violence and vote-rigging dominated the polls, with observers condemning the presidential election as a "charade". European Union monitors cited poor organisation, lack of transparency, significant evidence of fraud and disenfranchisement of voters.

A coalition of civil society observers called for the vote to be annulled and held again, but outgoing president Olusegun Obasanjo - who admitted the election was not perfect - called on the losers to use the courts for any complaints.

The run-up to the elections was marred by confusion over whether a number of state governor candidates who were accused of corruption by the Economic and Financial Crimes Commission could stand. In the presidential race, the candidacy of outgoing Vice President Atiku Abubakar - a powerful northerner also tainted by corruption allegations, who ran on an opposition ticket after falling out with Obasanjo - was approved only a few days before the vote.

The chaotic elections dented the image of Obasanjo, a retired general once hailed as a democratic hero after becoming the first Nigerian army ruler to hand over to an elected head of state in 1979.

The presidential poll represented the first handover of power between civilian leaders since Nigeria gained independence from Britain in 1960, but analysts said the flawed vote tarnished the transition.

During his rule, Obasanjo boosted Nigeria's regional role as a major actor in bodies including the African Union and mediator in west African conflicts. However, experts said the elections had undermined the country's credibility and could limit its future role as a regional broker.

While Obasanjo is an ethnic Yoruba from the Christian and animist southwest, his replacement, Yar'Adua, is a former governor of Katsina State in the north. In 1999, Obasanjo enjoyed the support of the political elite in the mainly Muslim, Hausa-speaking north - who ruled Nigeria for three decades following independence from Britain - because they saw him as a moderate who would not harm their interests.

However, with northern leaders eager for a turn in power, most of the major parties fielded candidates from the north in the 2007 elections.

In the oil-producing Niger Delta, where militant attacks are frequent, the rise to vice president of Goodluck Jonathan - a member of the delta's majority ethnic Ijaw group and former governor of Bayelsa State - was viewed as an opportunity to reduce poverty and violence.

Analysts agree that understanding violence in Nigeria means looking beyond superficial labels of religious and ethnic conflict. As the Internal Displacement Monitoring Centre (IDMC) observed in a February 2005 report, "the reality is usually more complex".

Often conflict erupts over an issue such as access to scarce resources or political tension, but polarises along Muslim/Christian or ethnic lines. It is then described as religious or ethnic, masking the underlying economic, social and political factors.

According to the IDMC, one of the key causes of communal conflicts in Nigeria lies in divisions between those who consider themselves indigenous to an area and those regarded as settlers. Where resources are scarce, this can lead to a build-up of resentment between the groups, in some cases, spilling over into violence.

One of the worst recent examples of this type of conflict occurred in 2004 in the buffer region between the predominantly Muslim north and Christian south, known as the Middle Belt.

This area was originally inhabited and controlled by tiny animist tribes that survived by farming. For centuries, Northern nomadic cattle herders have moved through the region and progressively settled there, in some cases establishing economic and political control, and taking land and power away from indigenous peoples.

As pressure on resources and the influence of fundamentalist Islam and Christianity have grown, relations between the two groups have become increasingly polarised.

Large-scale internecine killing began in Plateau state in 2001, and the conflict simmered for two years until an escalating series of tit-for-tat tribal militia attacks in 2003 and 2004.

In May 2004, Christian militia attacked Muslim Fulanis in the town of Yelwa, killing more than 600 according to the Nigerian Red Cross. The attack was followed several days later by deadly reprisals against minority Christians in the northern city of Kano.

Nigeria's first census in 15 years, conducted in March 2006, heightened tensions in some regions. Previous censuses provoked unrest between Nigeria's main ethnic groups, which have tried to use numerical superiority to claim a larger share of oil revenues and political representation. The results of several counts were discredited or even annulled.

Although questions on faith and ethnic background were excluded in 2006, at least eight people died in the southeast, where a separatist group, the Movement for the Actualisation of the Sovereign State of Biafra (MASSOB), tried to stop people from being counted.

MASSOB campaigns for southeastern oil-producing states to secede - as was attempted in 1967, sparking the bloody three-year Biafran War.

The census figures showed that the country's mainly Muslim northern states accounted for just over half of the country's population - roughly the same as in the previous census.

Another key factor behind civil unrest in Nigeria is widespread poverty - 70 percent of the population live on less than a dollar a day and one-third live below the national poverty line.

Unemployment runs high, particularly among young men, whose frustrations and anger are easily manipulated by politicians and religious leaders aiming to boost their power bases.

Economic hardship in northern states is likely to have fuelled a series of clashes in 2000, initially sparked by harsh sharia punishments handed down by Islamic courts, in which thousands of people died.

Nigeria's rich oil resources - it is the world's eighth-largest crude-oil exporter - have done little to alleviate poverty, even in the southern Niger Delta where the reserves are situated.

A 2006 briefing by the U.S. Council on Foreign Relations said few Nigerians have benefited since oil exploration began in the 1970s because the distribution of revenues has been undermined by corruption and mismanagement. Nigeria ranked joint 142nd out of 163 countries in Transparency International's 2006 Corruption Perceptions Index.

In 1995, the Nigerian government executed the writer and human rights campaigner Ken Saro-Wiwa and eight other members of the Ogoni ethnic community from the southern Niger Delta. They had campaigned against environmental damage by the oil industry and called for economic and social rights for the Ogoni people.

Their deaths caused international shockwaves and led the European Union to impose sanctions on Nigeria. Yet according to a report by Amnesty International, a decade later, "exploitation of oil in the Niger Delta continues to result in deprivation, injustice and violence".

Since late 2005, a group known as the Movement for the Emancipation of the Niger Delta (MEND), whose members belong to the region's dominant Ijaw tribe, has abducted numerous foreign oil workers and stepped up attacks on oil facilities. Militias have also carried out copy-cat abductions, robberies and oilfield invasions, seeking ransoms or benefits for their villages from oil companies.

Western multinationals operating in the area, including ExxonMobil, Chevron, Total and Royal Dutch Shell, are on a heightened state of alert, and thousands of foreign workers and their families have left the country. Oil output has fallen by a fifth.

The government has branded the militants "rascals" and oil thieves, but the Ijaw movement rejects this, arguing that it is fighting for justice. Its demands include the demilitarisation of the delta, basic services for local communities, the release of jailed activists and compensation for oil pollution.

The crisis in the Niger Delta is rooted in corruption and neglect by federal, state and local governments. They have collected billions of dollars of oil revenue but failed to provide basic services to the population. A 2007 report from Human Rights Watch detailed the misuse of funds by local officials and the harmful impact on primary eduation and basic healthcare.

Attempts by ordinary people to siphon oil illegally from pipelines frequently end in tragedy. In May 2006, more than 150 people were killed when a high-pressure pipeline exploded in a Lagos suburb, allegedly during an attempt to tap into the pipeline. At least 2,000 people have died in similar incidents in recent years.

Yet rather than cracking down on rampant corruption and helping improve local services, the government has militarised the Niger Delta region to protect oil exports and overlooked routine human rights abuses by its soldiers.

Western oil companies have been targeted by local militia because of their association with the government. Some human rights groups have called on the oil industry to help improve the situation by boosting transparency surrounding their payments to the government and providing direct benefits to local people.

Analysts say the key to solving the crisis will be improved governance, free and fair elections, and public provision of services such as water, electricity, roads, public transport, schools and small business development.

The April 2007 elections elevated Goodluck Jonathan, an Ijaw and former governor of Bayelsa State, to the position of vice president. This was seen by some as an opportunity to reduce poverty and violence.

Right campaigners said Jonathan had a chance to open new lines of dialogue with the delta's militants, but would have to move quickly in addressing their political and economic grievances.

Before the elections, a MEND spokesman told Reuters the group anticipates a long guerilla war, which it hopes would lead to external intervention, eventually forcing the Nigerian government to make concessions.

One of the most serious - and largely hidden - consequences of Nigeria's ongoing violence is the displacement of hundreds of thousands of people from their homes.

In a March 2006 report, the National Commission for Refugees (NCR) put the total of internally displaced persons (IDPs) at more than three million over the past seven years. It said the problem was getting worse and appeared to be a permanent feature of society.

However, the true number of IDPs in Nigeria is disputed. The IDMC declines to give an estimate, describing the issue of numbers of IDPs as "very problematic". The main reason is the absence of a registration system, and thus a lack of data.

In April 2006, a presidential advisor on migration and humanitarian affairs said estimates for the number of IDPs in the country varied from 500,000 to millions.

The picture is further confused by complex patterns of movement. Many displaced Nigerians seek refuge with their families, friends or communities of the same ethnic group. Once violence has subsided, some go home or resettle nearby but others head to different areas, making it hard to distinguish between those fleeing violence and economic migrants.

Another feature of displacement in Nigeria is that it is often short term. In late February 2006, a wave of sectarian violence erupted in mainly Muslim towns in the north and in the southern city of Onitsha, fuelled by global controversy over cartoons of the Prophet Mohammad published in a Danish newspaper.

This resulted in over 100 deaths and the displacement of as many as 50,000 people. But many took refuge in police and army barracks temporarily, and are likely to have returned home.

According to IDMC analyst Claudia McGoldrick, the NCR statistics may be skewed because of ongoing political wrangling between that body and the National Emergency Management Agency over responsibility for IDPs.

While she said it was impossible give an accurate figure, "with numerous outbreaks of communal violence, it is fair to say that hundreds of thousands have been displaced in the past few years."

One of the largest movements of people in recent years was seen after the violence in Plateau state in 2004, caused by conflict over land between Muslim nomads and settled farmers.

Estimates put the number of IDPs stemming from this crisis at between 200,000 and 260,000. Although the majority were taken in by host communities, around 60,000 sought refuge in camps in neighbouring Bauchi and Nassarawa states.

In the wake of the 2004 crisis in Plateau state, the Nigerian government appealed to the United Nations for financial support to deal with displacement. Several U.N. agencies set up a joint task force to work with state governments to address short-term needs.

The Nigerian Red Cross Society was the first organisation to reach Yelwa, the scene of the worst violence, and is widely regarded as the country's most efficient humanitarian body. Overall, the international response was limited, because most donors did not regard the situation as a real humanitarian emergency.

According to IDMC's McGoldrick, while the Nigerian government does have the financial resources to deal with such crises, in the Plateau case, the response was "completely muddled". Thus she argues that donors should support training to improve government capacity to tackle displacement.

IDMC also highlights the need for sustained support to help displaced people return and reintegrate - not only in terms of rebuilding homes and infrastructure, but also peace and reconciliation initiatives.

"All too often in Nigeria, once an outbreak of conflict has died down, humanitarian assistance to those displaced becomes virtually non-existent," its 2005 report stressed.

Despite the relatively low level of international humanitarian engagement in Nigeria, several international NGOs have worked on alleviating the consequences of displacement, including medical aid agency Medecins Sans Frontieres and Catholic Relief Services.

A current priority for international NGOs operating in Nigeria is HIV/AIDS. According to the U.N. Programme on HIV and AIDS (UNAIDS), 3.9 percent of adults aged between 15 and 49 were HIV positive in 2005.

Nigeria's 2004 U.N. Development Report warned that the government's limited capacity to respond could lead to an infection rate of 15 to 25 percent by 2010, and urged measures to stop the spread of the virus among young people.

In December 2006, the Dutch government launched a health insurance scheme aimed at providing thousands of low-income workers in the informal sector with cheap, basic healthcare.

The United Nations Development Programme in Nigeria has four key programme areas: governance and human rights; poverty reduction; energy and the environment; and HIV/AIDS.