* Fighting spreads between government forces and rebels

* Plight of civilians alarming-UN

* Tens of thousands left homeless

By Richard Meares

LONDON, Sept 17 (Reuters) - A month-long conflict between Yemeni government forces and rebels has spread, taking the plight of civilians to alarming levels, with hungry people stranded in the open, on the sides of roads and under bridges, the United Nations said.

However, a U.N. appeal for emergency funds to help up to 150,000 displaced people in the north of the poorest Arab nation has so far met with silence.

A new wave of fighting - the "sixth war" in an intermittent five-year-old conflict - erupted a month ago in the mountainous north between rebel Shi'ite Muslims and government forces trying to impose central authority.

U.N. agencies estimate this has added up to 50,000 people to the 100,000 or so already left homeless by earlier rounds of the fighting. Most are women and children and many have moved for the second or third time as the arc of fighting widens across Saada governorate into the neighbouring region of Amran.

"The humanitarian situation of the civilians in the conflict zone reached alarming and unprecedented levels after one month due to insecurity, lack of humanitarian access, and government banning.

The humanitarian community had repeatedly appealed for opening humanitarian corridors to allow access and delivery of urgently needed relief items," the U.N refugee agency UNHCR said in a report received by AlertNet on Tuesday.

"Saada Governorate has been cut off from the rest of the world for more than one month now. There has been no water or electricity in Saada city since 12 August and food reserves are running out.

High temperatures during the day and heavy rains at night have left civilians, most of them observing the Ramadan fasting period, in dire need for shelter, food and clean water."

The road from and to Saada was extremely dangerous because of fighting, for which no reliable casualty figures have been made available. Numerous checkpoints and roadblocks, a state of emergency and a 12-hour curfew were further isolating the area.

"Most of the displaced are stranded and dangerously exposed to the fighting as they are unable to reach safer areas. Mines are also reported to be hindering safe passage for those attempting to flee. The only back exit road through Al Jawf governorate is also blocked subjecting civilians to risky journeys through mountainous deserts."

Aid agencies sent a mission through neighbouring Saudi Arabia to assess how to help up to 30,000 displaced people who had fled north to a border area at Baqim - only to find that the conflict had already engulfed that area too.

DIRE NEEDS

"Hundreds of families are currently living in schools or even in the open, on the sides of roads, and under bridges, with little to feed on.

They are in dire need for shelter, food, and water. Despite the agency's readiness to dispatch needed assistance at the first signal, the authorities have not yet allowed access at this moment to the area due to insecurity," UNHCR said.

Agencies have been more able to reach people fleeing south towards the capital Sanaa, about 300 km (180 miles) from Saada. The UNHCR began establishing one of its new camps in the Amran region last week but security there has deteriorated too, forcing work to be suspended.

Some displaced people said they witnessed fierce fighting, heavy attack and airstrikes.

"They have spent three to five days walking in the desert, taking mountainous roads on foot due to the blockage of main roads, before reaching the camps. They arrived traumatised and exhausted. The majority of them are women," the report said.

Before the latest round of fighting the U.N. food agency had to cut rations to previously displaced Yemenis because of a lack of funding. On Sept 2 the United Nations called for $23.75 million in an emergency appeal to address the humanitarian situation in Yemen.

"No funds were earmarked to date," the UNHCR said of the $5 million it has sought as part of that appeal.

Yemen also faces secessionist sentiment in the south and a new al Qaeda campaign that has staged deadly attacks over the past three years.

News reports and a refugee at the improvised camp at Adi in Harf Sufyan, at the centre of fighting, said around 87 people died in an army raid on the camp on Wednesday afternoon.

"The camp was taken by surprise by the air force bombing them," the source, who did not want to reveal his name, said.

The independent website News Yemen said 85 people died.

"An air raid hit them in the area when they were sleeping under trees and plastic awnings," it said, citing witnesses. It said the air force then staged a second raid on the camp.

A government statement said there was no refugee camp in the area but did not confirm or deny the incident.

The government says ordinary residents there do not support the uprising by rebels referred to as Houthis after the clan of their leaders.

People in the main town Saada are trapped by shelling and street-fighting in homes with barely any water, power or communications. Food is scarce and prices spiralling out of reach.

(Editing by Samia Nakhoul)

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