About 35,000 people may have been affected by fighting in Sudan's southern Darfur region, but authorities have told aid agencies it is not safe for them to access the area, the UN said on Monday.
Residents fled after rebel attacks and "possible" air strikes around the towns of Muhagiriya and Labado, about 100 kilometres (62 miles) east of the South Darfur state capital Nyala, international peacekeepers have said.
"Apparently the authorities deemed it too insecure for aid agencies to go there," said Damian Rance, of the UN's Office for the Coordination of Humanitarian Affairs.
"We assume that there's a population of more than 35,000 people that has been affected," he added, but agencies would first need to reach the area and assess the situation.
Thousands of the displaced residents sought protection around bases of the African Union-United Nations Mission in Darfur (UNAMID), the peacekeeping mission said.
The Sudan Liberation Army's Minni Minnawi faction still claimed to be in control of Muhagiriya and Labado on Monday, an unusual nine-day occupation by insurgents who have traditionally staged hit-and-run attacks.
Sudan's army spokesman could not be reached but last Wednesday Defence Minister Abdelrahim Mohammed Hussein said troops expected to move soon against the rebels.
Muhagiriya and Labado are located strategically along main supply routes used by traders and aid workers to move supplies from Khartoum through East Darfur to South Darfur, OCHA said in its latest weekly bulletin.
Sudan's government on Sunday warned over insecurity in South Darfur, where rebels said they had also seized an army garrison and the top local official has been replaced by a retired general.
Minnawi rebels said they killed government troops on Sunday in an attack on the Donki Dreisa base, about 50 kilometres south of Nyala.
Last week, the same faction claimed to have moved through Ishma village, 30 kilometres east of the state capital.
Vice President Ali Osman Taha "confirmed the importance of security and stability", justice and rule of law in South Darfur, the official SUNA news agency reported on Sunday.
Taha made the comments during a meeting with the new South Darfur governor, retired general Adam Mahmoud. He replaced a civilian, Hamad Ismail, who was removed on Friday.
"I am going to make my best effort to have security and stability," SUNA quoted Mahmoud as saying.
In February a United Nations panel of experts said it had collected testimonies of insecurity, including "growing crime inside towns such as Nyala".
Humanitarian and development agencies still face "access restrictions that compromise donors' ability to plan and monitor their programmes", the European Union told a conference in the Gulf state of Qatar last week.
The conference backed a six-year strategy aiming to move Darfur away from food handouts and other emergency aid, and laying the foundation for lasting development.
The EU and other donors pledged a total of about $1 billion to support the strategy.
Sudan committed itself to $2.65 billion and said it would "enable access and free movement" of its development partners and non-governmental organisations.
Ethnic rebels in Sudan's far-western Darfur region rose up against the Arab-dominated Khartoum government in 2003.
While the worst of the violence has long passed, rebel-government battles continue but instability has been complicated by inter-Arab fighting, kidnappings, carjackings and other crimes, many of them suspected to be the work of government-linked militia and paramilitary groups.