The biggest offensive in years by Nigerian forces against militant groups in the heartland of Africa's largest oil and gas industry has raised fears for the safety of civilians caught up in the fighting.

Here are key questions and answers about the humanitarian situation in Nigeria's Niger Delta.

What's the latest on the fighting?

Nigerian security forces last month launched major operations against rebels in Delta state, bombarding militant camps from the air and sea and sending three battalions of soldiers to hunt down insurgents in surrounding communities.

Most of the operations were focused around state capital Warri and have since calmed down, says the International Committee of the Red Cross (ICRC) and the National Emergency Management Agency (NEMA).

But NEMA, which was called in by the state government to help cope with the crisis, says the continued presence of the military was causing apprehension among the local population, with many men going into hiding.

Rivers and Bayelsa, the two other main states in the Niger Delta, have been relatively quiet, although Nigeria's military said on Wednesday the navy had killed seven militants in a gun battle along the Isaka-Bakan waterway near Bonny in Rivers state.

The operations come after years of instability in Niger Delta, where a loose coalition of militant groups have declared an "all-out war" against the military and launched repeated attacks on oil and gas installations.

How many civilians have been displaced and where are they?

Aid agencies and government officials say the number of people uprooted by the offensive in Delta state is unclear, with figures varying from 1,000 to 5,000.

NEMA says 3,500 people have been displaced around the settlement of Ogbe-Ijoh in the Warri South West local government area.

An analyst at the Geneva-based Internal Displacement Monitoring Centre said precise figures were hard to come by because most people would probably seek shelter in hospitals and schools and no registration process existed.

NEMA says many of the displaced are staying with host families and relations, although some regularly visit a government-run displacement centre set up in a primary school in Ogbe-Ijoh to receive food and other supplies.

The ICRC says communities in the Niger Delta are traditionally hospitable, and that families with the means to help are likely to absorb as many vulnerable people as possible on a short-term basis. But many are still left to fend for themselves.

Are aid agencies able to access the area?

Initial reports suggested the military was not allowing access to the operation zone but a spokesperson for the ICRC said the military's Joint Task Force had given full access to the Nigerian Red Cross and NEMA.

The ICRC and Nigerian Red Cross have provided emergency assistance to more than 300 women and children sheltering at the primary school in Ogbe-Ijoh, 30 km west of Warri. The Nigerian Red Cross has also been able to reach some people along Delta state's waterways.

The big challenge is accessing people in outlying areas crisscrossed by creeks and rivers and covered by thick mangrove forests. Many young men are in hiding, fearing arrest or interrogation by the military as it hunts down suspected militants.

NEMA has sent a doctor in a small boat along waterways to treat some of the displaced, but it described access as difficult.

What are the longer-term prospects?

Relief groups say the humanitarian situation could become critical if the offensive is not over soon.

Medical care is a priority. IDMC analysts warn that conditions in overcrowded schools and other public buildings could deteriorate as water and sanitation systems are overtaxed.

One local NGO, the Centre for Social and Corporate Responsibility, says communities that depend on fishing for a living face economic pain if the crisis stretches on.

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