The attack which targeted a convoy carrying fuel in the Kuma area around 80 kilometres (50 miles) northeast of Darfur's main town of el-Fasher was condemned by the African Union mission in Sudan.

An African Union statement said: "The African Union mission in Sudan [AMIS] condemns in the strongest possible terms this outrageous attack against its forces in Darfur and intends to carry [out] a thorough investigation."

The attack happened in an area that is under the control of rebels who did not sign the May peace deal, but the AU said it was not clear who the attackers were.

Only one of the three rebel factions involved in negotiations signed the AU-brokered peace deal in May. Tens of thousands of Darfuris have protested against the accord, saying they want more compensation for war victims, more political posts and a monitoring role in disarming Arab militia, known as Janjaweed.

This week the AU expelled non-signatory rebel representatives from its camps, saying it could no longer guarantee their safety because the government called them terrorists after some of the groups formed a new alliance which has attacked the government since the May deal.

The non-signatories said that in doing this the AU had terminated a shaky truce signed in April 2004 by all the parties to the conflict.

Tens of thousands of civilians have been killed and 2.5million forced from their homes since mostly non-Arab rebels took up arms in early 2003 accusing the central government of neglect.

Around 7,000 AU soldiers are struggling to stop the violence, which has been on the rise since May.