LONDON (AlertNet) - A flare-up of fighting in north Yemen has sent some 35,000 people fleeing from their homes in the last fortnight alone, the United Nations said on Friday.

Trapped in a remote mountain area with no air links and access roads blocked as government forces take on Shi'ite rebels, people were taking great risks to get out, even paying smugglers to rescue them.

Yemeni forces have used air strikes, tanks and artillery in an offensive this month against the rebels led by Abdul-Malik al-Houthi, the latest round in an intermittent five-year-old conflict.

"Today it is a real tragedy," the U.N. refugee agency UNHCR's Yemen representative Claire Bourgeois said in a statement.

"Some internally displaced people are displaced for the second or third time. They were already living in precarious situations for months or even years and now they have to go through the drama all over again."

U.N. Secretary-General Ban Ki-moon appealed this week for combatants to facilitate the delivery of humanitarian assistance to trapped civilians.

The UNHCR quoted people fleeing the closed-off area as saying the situation seemed critical in the main town there, Sa'ada city, and further north towards the Saudi border.

UNHCR, which has been supporting some 100,000 people affected by the fighting in the north since 2007, appealed to donor governments for another $5 million for this latest emergency to help pay for camps for the next four months.

The plight of the civilians caught up worsened last month when the U.N.'s food agency WFP said it had been forced to halve monthly rations for all 100,000 due to lack of funds.

The UNHCR's annual budget for Yemen is $22 million - but that also has to cover help for some 150,000 refugees and asylum seekers who have made the dangerous journey to Yemen across the Gulf of Aden, most from Somalia.

It stressed its figure of 35,000 more people leaving their homes was an estimate.

The International Committee of the Red Cross (ICRC), working in the area, told AlertNet it could not yet provide a reliable estimate of the number of new internally displaced people (IDPs).

"The ICRC and the Yemen Red Crescent Society have by now registered a total of about 12,000 newly arrived IDPs in the Sa'ada City, and the Baqim and Sahar districts of Sa'ada Governorate. Registrations are continuing," said a spokesman in Geneva.

"The two organisations have also begun distributing essential household items and some IDPs have been provided with locally bought food supplies. The overall situation remains difficult."

SECESSIONIST VIOLENCE

The resurgence of the conflict compounds the troubles of a country already grappling with secessionist violence in the south and a growing threat from al Qaeda militancy.

The rebels, adherents of the Zaydi branch of Shi'ite Islam -- a strongly tribal minority in mostly Sunni Muslim Yemen -- oppose Yemen's close ties with the United States and say they are defending their villages against government oppression.

Foreign Minister Abubakr al-Qirbi blamed the rebels on Monday for the surge in fighting, saying they had ignored the president's declaration a year ago of an end to the war and all invitations since to renew dialogue.

"Over the last six months they have tried to expand their influence," he told Reuters. "In the end, things exploded and we have this war going on now."

He declined to give casualty figures for the fighting in an area barred to journalists and diplomats and where the few aid agencies operating are constrained by security concerns.

Last week 15 Yemeni Red Crescent workers, including doctors and nurses, were temporarily kidnapped from IDP camps in Sa'ada governorate.

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