About 30 civilians have been killed in an attack by gunmen on a convoy carrying medical and relief supplies in the Darfur region of western Sudan.
"Some people were shot, others were burned to death," said United Nations spokeswoman Radhia Achouri.
The attack appears to be the work of the pro-government Janjaweed militia, the BBC's Jonah Fisher in Sudan says.
About 200,000 people have been killed in Darfur since 2003, when rebels took up arms against the government.
Since then, an estimated two million people, mostly black Africans whose villages have been attacked by the Arab Janjaweed, have fled their homes.
The Sudanese government has rejected a UN Security Council resolution authorising the deployment of UN troops and police to Darfur.
A small force of 7,000 African Union (AU) peacekeepers has struggled to protect civilians in the absence of a UN contingent.
Our correspondent says that in the last few months, the Janjaweed have been mobilised along the border with Chad, destroying villages considered loyal to rebel movements.
The Sudanese government denies accusations that it is backing the militias to put down the uprising.
'Rebels to blame'
Saturday's attack on the convoy took place near Sirba, close to the Sudan-Chad border.
The UN and the AU did not identify who was behind it.
An international aid worker told the Associated Press news agency that Janjaweed members attacked the convoy with rocket-propelled grenades, then killed survivors.
The governor of West Darfur said the attack was carried out by rebel groups who refused to sign the May peace agreement reached between Khartoum and the main faction of the Sudan Liberation Movement.
"The raid was perpetrated by the rebels from the National Redemption Front, who are committed to a military escalation," Mohammed Yusef al-Tulib was quoted as saying by the state-run Suna news agency.
The AU, which dispatched an investigation unit to Sirba, said its team was able to leave the area after angry villagers initially prevented them from departing.
Local residents insisted that the AU team should take pictures of the bodies, and stopped them from flying out by helicopter until they did so.
The attack came as a coalition of human rights groups held a day of protests around the world against sexual violence in Darfur, where they say thousands of women and girls have been raped by forces that support the Sudanese government.
US envoy Andrew Natsios is currently in Sudan, attempting to persuade Khartoum to accept a joint AU-UN peacekeeping force for Darfur.