Some 30 civilians were killed by militiamen who ambushed their refugee convoy in western Darfur, and African Union peacekeepers investigating the attack were taken hostage by rioters, the AU and the UN said.
The United Nations said armed men on horseback ambushed the civilian truck transporting medicine and relief items between the towns of El Geneina and Sirba.
The exact number of civilians killed could not be confirmed, but was estimated at around 30 people, said Radhia Achouri, spokeswoman for the UN mission in Sudan.
"Some of the passengers were shot by the attackers and others were burnt to death during the attack," the UN said in a separate statement Sunday.
An international aid worker said the assailants were government-backed janjaweed - members of an Arab tribal militia - and that they attacked the convoy with rocket-propelled grenades, then executed survivors. He spoke on condition of anonymity because of restrictions the Sudanese government has placed on aid workers. AU spokesman Noureddine Mezni said rioters angry over the deaths were holding captive a helicopter crew and eight AU peacekeepers who were in Sirba to investigate the incident.
The AU base in El Geneina was also being attacked, Mezni said. Sudanese police transported the bodies of the victims, all ethnic African refugees from the Erenga tribe, to a refugee camp on the outskirts of El Geneina, and demonstrations erupted around the town, the aid worker said.
A UN official in Darfur said rioting and sporadic shooting took place in the town on Sunday.
He said he did not know who was attacking the AU base, but said he did not believe it was the janjaweed.
The airport was closed and all UN staff and aid workers were grounded, the official said on condition of anonymity because he was not authorized to talk to the media.
A 4 pm (1300GMT) curfew has been imposed in El Geneina, but the UN had no plans to evacuate staff for now, he said. Some 135 UN staff, diplomats and aid workers have already been evacuated from the North Darfur capital of El Fasher, after at least 10 people were killed in a week of janjaweed attacks there.