Renewed aerial bombardments by the Sudanese government in West Darfur are endangering tens of thousands of civilians, said the United Nations Tuesday, calling on all parties to exercise restraint in the face of the flare up in fighting between the army and the rebels.

Rebel successes over the last few weeks in West Darfur have been met by government air strikes that have sent thousands of civilians fleeing their homes, many across the border into neighboring Chad, which facing its own instability.

U.N. Secretary-General Ban Ki-moon "is extremely concerned by the renewed violence in West Darfur," U.N. deputy spokeswoman Marie Okabe said in New York, citing in particular the bombing of Aro Sharow camp for the displaced Monday and Tuesday.

"Additional reports from Darfur indicating that government and militia forces are amassing in the Jebbel Moon area of West Darfur are a worrying sign that there will be continued hostilities in the area," Okabe said.

For his part, the U.N. Undersecretary General for Humanitarian Affairs John Holmes noted Monday that "the consequences for 20,000 civilians in this area could be disastrous."

The latest round of violence in Darfur, where 200,000 people have been killed and 2.5 million displaced since rebels took up arms against the central government, follows the renewed fighting between Khartoum and the opposition Justice and Equality Movement at the end of 2007.

According to the U.N., which jointly operates the UNAMID peacekeeping force in Darfur with the African Union, some 57,000 civilians have been displaced by the rebel offensive and subsequent government counteroffensive and its aerial bombardments.The bombings have caused at least 10,000 people to flee across the border from West Darfur to Chad since the aerial bombings there earlier this month, the U.N. High Commissioner for Refugees said Tuesday.

Reports of fresh bombings overnight forced the U.N. to temporarily withdraw its staff from the border area for safety reasons, spokeswoman Jennifer Pagonis told reporters in Geneva.

Refugees who carried an injured woman across the border Monday night said she had lost both her legs when a camp for displaced Darfurians was bombed, Pagonis said. The woman later died.

"We have no confirmation or further details of the alleged bombing raid but bombing could be heard from Birak," on the Chadian side of the border, Pagonis said.

Chad is home to some 300,000 refugees from Darfur who have fled years of fighting there between government-backed militias and rebel groups.

Chad has accused the refugees of fomenting instability in the country.