KHARTOUM, Sudan — Sudanese government forces attacked one of Darfur's largest refugee camps at dawn Monday, killing dozens of civilians, according to rebels and a witness in the camp.

The United Nations said it was "gravely concerned" about the situation inside the Kalma camp in southern Darfur after it received reports that Sudanese police vehicles surrounded it and subsequent attacks caused "injuries and deaths of civilians."

The U.N. did not provide a death toll. But a resident of the camp, Mandela Abdullah Mohammed, told The Associated Press by telephone that he counted 32 dead bodies, including several women and children.

More than 50 vehicles "packed with armed men wearing police and security forces' uniforms...hit us with rocket-propelled grenades and machine guns," Mohammed said.

Government officials did not immediately answer calls seeking comment.

The assault comes as Sudan's president faces genocide charges for alleged government-backed attacks on ethnic Africans in the country's troubled western region. Up to 300,000 people have died and more than 2.5 million have been displaced since a rebellion began in Darfur in 2003.

The International Criminal Court is expected to decide whether to issue a warrant for his arrest within months.

Kalma, which sits about 15 miles from the South Darfur capital of Nyala, is home to about 90,000 residents.

Sudanese soldiers stormed the camp, a sprawling mix of mud huts and scrap-plastic tents, and immediately opened fire on civilians, said Yahia Bolad, spokesman for the Sudan Liberation Movement, a Darfur rebel group.

He told AP by telephone from London that some 50 residents were killed and 100 injured. Bolad said his reports came from fellow rebel leaders on the ground, but the information couldn't be independently verified.

Troops raided the same camp two weeks ago and confiscated weapons, he said.

"The government sent a strong military force and attacked the camp with the intention of killing civilians," Bolad said.

In its report about the attack, U.N.-operated Miraya radio quoted a Kalma camp spokesman as saying 48 people had been killed. Later Monday, hundreds of refugees staged demonstrations against Sudan's government, it said.

At least 65 wounded — more than half of them women and children — were admitted to a nearby clinic run by Doctors Without Borders, the aid group said in a statement.

The U.N. said it was trying to get aid workers in to evacuate the wounded. It said such attacks "severely threaten the safety and security of civilians who have a right to protection."