A fragile calm has been restored in the south Sudan town of Malakal after days of heavy fighting between government forces and former rebels that killed more than a dozen people, the United Nations mission in Sudan and aid workers said Thursday.

Skirmishes between the Sudanese People's Liberation Army and government forces began late last week inside Malakal, which lies about 400 miles south of Khartoum, the Sudanese capital.

Aid workers from Malakal said the fighting began when government forces tried to kill a local SPLM commander. On Monday evening, the SPLM retaliated against the town's army commander, and about a dozen fighters from each side were killed, aid workers said. Clashes evolved into large-scale battles on Tuesday, and the situation remained tense even after U.N. peacekeepers were deployed in the town on Thursday, they said.

They spoke on condition of anonymity because of the sensitivity of the issue.

In New York, U.N. Secretary General Kofi Annan was "deeply concerned about the reports of heavy fighting" between the army and the former rebels, his spokesman Stephane Dujarric said Wednesday.

A January 2005 peace deal ended 21 years of civil war between north and south Sudan, but the situation in Malakal has remained volatile. The town, which counts about 150,000 people and lies close to Sudan's richest oil fields, is located next to the north-south boundary.

Nonessential U.N. staff and almost all aid workers were evacuated from the town on Wednesday. The U.N. has some 10,000 peacekeepers in south Sudan to monitor the peace agreement and help reconstruct the ravaged region.

The clashes in Malakal "constitute a serious violation of the security arrangements" under the peace deal, Annan's spokesman said.

A joint military delegation with high level officers from the SPLM, the Sudanese army and the U.N. mission's force commander had arrived in Malakal to assess the situation, the U.N. said.

The Sudan Tribune and Al-Sahafa newspapers reported that nearly 100 people had been killed in the clashes and over 300 wounded but that figure could not be independently verified.

The U.N. said it could not confirm any death toll, but would investigate causalities in what appeared to be one of the gravest breaches of the 2005 cease-fire.

An aid worker evacuated Wednesday from Malakal said he suspected dozens had been killed.

"This wasn't little guerrilla shooting, this was war," said Renaldo Fiocco in a telephone interview.

Fiocco, who works for the French aid group Medecins du Monde, said the worst battles occurred in the center of the town, with artillery barrages, mortar rounds and hours of shooting. He said he saw government troops in armored vehicles clashing with former rebels in pickup trucks with heavy machine guns.

Radhia Achouri, a spokeswoman for the U.N. in Sudan, said U.N. peacekeepers were deployed in the town Thursday and that calm was restored.

The Sudanese military was not immediately available for comment.