Government allied troops launched a new attack on a key rebel-held zone in southern Darfur, as the U.N. and aid groups on Monday advised some workers to temporarily leave the regional capital because of insecurity.

Rebels said the Sudanese army and its allied janjaweed militia unleashed a large offensive on the South Darfur town of Muhajeria. The U.S. Embassy in Khartoum also said Monday that violence appeared to spreading to Muhajeria.

The town was considered neutral because it is a base for fighters from the Sudan Liberation Army faction belonging to Minni Minawi, the sole Darfur rebel chief who signed a peace agreement with the government last year.

Minawi's leadership was not immediately available for comment, but other splinter SLA chiefs said a large clash was taking place.

"There are heavy aerial bombings happening right now," said Suleiman Jamous, a leader of the faction known as SLA-Unity.

Sudan's government denies widespread international accusations it backs the janjaweed, who are blamed for the worst atrocities in the more than 4-year-old conflict that has killed over 200,000 people and chased 2.5 million from their homes.

The office of the Sudanese military spokesman did not answer calls from The Associated Press on Monday, but the army regularly denies it conducts military flights, which are banned over Darfur by several U.N. resolutions.

The attack in Muhajeria comes about a week after a separate rebel attack at an African Union base near the town of Haskanita that left 10 AU peacekeepers dead.

Jamous' SLA-Unity faction, which has not agreed to peace with the government, was suspected in the Sept. 30 attack near Haskanita, which is located about 60 kilometers (37 miles) east of Muhajeria. Jamous on Monday denied his group was involved in the attack but said his rebels were nearby when it took place.

Since the attack at the AU base, about 15,000 civilians have fled from around Haskanita, which was burnt to the ground since coming under government control last week.

In a statement released Monday, the U.S. Embassy strongly condemned the destruction of Haskanita.

"While all the details are not clear, the town (of Haskanita) which was under the control of the government of Sudan, was burnt down except for a few buildings," the statement said.

"The violence seems to be spreading to neighboring Muhajeria," it said.

Echoing calls from the United Nations, the U.S. Embassy also pressed for all parties to "immediately end the cycle of violence" in Darfur and commit to a cease-fire ahead of new peace talks between the government and rebel groups scheduled to take place later this month in Libya.

With violence in the war-torn region continuing, humanitarian groups and U.N. aid agencies advised nonessential staff to temporarily leave the South Darfur capital of Nyala.

"The situation isn't good," said Michael Arunga, the spokesman in Nyala for the U.S.-based Christian aid group World Vision, which was the largest humanitarian group operating around the town.

Arunga said nonessential staff were leaving, but five of the group's international aid workers would stay to maintain "lifesaving activities" including food distribution and medical help in the sprawling refugee camps that surround Nyala.

Orla Clinton, the spokeswoman for the U.N.'s Office for Coordination of Humanitarian Affairs, insisted the departures of aid workers were a temporary measure until the situation cooled off.

"There's no evacuation. There's an advisory for nonessential staff to go on leave," Clinton said. It was not immediately clear how many people the U.N. had advised to leave.

Though long-considered Darfur's safest town, Nyala has witnessed a spree of attacks on aid workers and other violence in recent weeks. At least six people were killed inside the town last week when a brawl between government soldiers and ex-rebels belonging to Minawi spiraled into a gunbattle.