At least 20 civilians have been killed in an attack in the Darfur region of western Sudan, the African Union (AU) and United Nations say.
A foreign aid worker quoted by AP news agency said pro-government Janjaweed militiamen had attacked a refugee convoy with rocket-propelled grenades.
He added that they had then set fire to the bodies.
News of the attack came on the Global Day for Darfur, called to protest at the situation in the region.
At least 200,000 people have died there in the past three years. An estimated two million people, mostly black Africans whose villages have been attacked by the Arab Janjaweed militia, have fled their homes.
The Sudanese government denies accusations it is backing the militias to put down an uprising launched by Darfur's rebel groups in 2003.
A small force of 7,000 African Union peacekeepers has struggled to protect civilians in the absence of a UN contingent.
Noureddine Mezni, AU spokesman in Sudan, said that in this latest attack, gunmen had attacked a vehicle near the village of Sirba, close to the border with Chad.
He said the AU had sent an investigation unit to Sirba, and angry villagers had later attempted to prevent the team from leaving.
"The African Union is under threat by the local population in Sirba. There are efforts now to allow the team to leave the area," he told Reuters news agency.
"We appeal to everybody to know that the AU is in Darfur to help the people and restore stability and reach peace. We are not in Darfur to be part of the conflict."
Armed men riding on horses and camels attacked Sirba in November, killing up to 30 villagers and wounding 40.