"As long as this insecurity continues, the international community cannot provide the assistance that is so desperately needed by hundreds of thousands of people," Antَnio Guterres, the UN High Commissioner for Refugees (UNHCR), said in a statement issued on Thursday in New York.

Ten other people were seriously wounded when armed men attacked the camp sheltering thousands of internally displaced persons (IDPs) on Wednesday.

According to the UNHCR, initial reports indicated that a group of 250 to 300 "armed Arab men on horses and camels" attacked Aro Sharow Camp, sending thousands of residents fleeing into the insecure countryside.

The attackers reportedly burned some 80 makeshift shelters, about one-quarter of the camp's households, UNHCR said.

Aro Sharow is 16 km north of the town of Saleah. Between 4,000 and 5,000 IDPs are believed to occupy the camp, and most of them reportedly fled into the surrounding countryside. The nearby village of Gosmeina was also attacked and burned, according to reports received by UNHCR.

"The government of Sudan has a responsibility to ensure security for all of its citizens," Guterres said.

The UN Mission in Sudan (UNMIS) also expressed alarm at the deteriorating security situation, saying villages had also been attacked in recent weeks.

The Special Representative of the UN Secretary-General to Sudan, Jan Pronk, was "deeply concerned about the upsurge of violence in Darfur at a time when every effort is being exerted to speed up a peaceful solution to the conflict in the region in the framework of Abuja talks", UNMIS said in a statement. It was referring to peace talks held on Friday in the Nigerian capital, aimed at ending the crisis in Darfur.

Pronk met on Thursday with the special representative of the chairperson of the African Union (AU) Commission for Sudan, Baba Gana Kingibe, to discuss the incident. The AU has several thousand monitors on the ground in Darfur.

Wednesday's attack followed a series of incidents of insecurity throughout Darfur.

The UNHCR said it was concerned that the deteriorating security situation was preventing the provision of vital aid to tens of thousands of IDPs and could prompt them to flee again, possibly to neighbouring Chad, which already has to cope with some 200,000 refugees from Sudan.

UN Emergency Relief Coordinator Jan Egeland said on Wednesday the situation in Darfur had become so dangerous that relief agencies had temporarily suspended operations in some areas.

The conflict in Darfur pits Sudanese government troops and allied militias like the Janjawid against two main rebel groups, the Sudan Liberation Movement/Army (SLM/A) and the Justice and Equality Movement, which claim to be fighting the marginalisation of their region by the Khartoum-based government.

According to the UN, at least 2.9 million people continue to be affected by the conflict, of whom 1.85 million are internally displaced or have fled to neighbouring Chad.

Meanwhile, Rwanda began airlifting some 538 troops to Darfur on Friday as part of their contingent of the African Union Mission (AMIS) in the country, deployed to help bring peace to the troubled region.

The troops join the 1,750-strong Rwandan contingent already in Darfur. AMIS has at least 5,000 troops in the region, but is expected to increase this number to 7,500 - its target strength.

"We were requested to provide a brigade and this is the last group which will be rotated after every six months," Lt-Col Charles Karamba, the director of research and development in the Rwanda Defence Forces, told reporters in Kigali on Friday.

Two US Air Force aircraft airlifted 80 Rwandan soldiers and plan to carry the remainder over the next one week. The 538 troops form the last batch of Rwandan troops requested by the AU.

Rwanda has been among the top contributors of troops to AMIS, which has been credited with helping calm the situation in some areas of strife-torn Darfur, allowing for the delivery of humanitarian aid to the region's war-affected populations.

"The AU force has helped to establish more stability," Pronk told the UN Security Council in July. "They have done an admirable job, highly professional, with much dedication."

However, AU recently told IRIN that the mission faced a funding crisis and would soon be unable to pay its troops' salaries.

"Everyone knows this mission is important and we think the international community will support us, but they need to do it soon because the money is fast running out," an AU official, who requested anonymity, said in August. "Within three months we will not be able to pay the wages of our troops who are on the ground there."