By Martin Plaut
BBC Africa analyst
Exactly who carried out the raid on the African Union base in Haskanita is unclear, but there are suggestions that it was a group called SLA-Unity.
About 7,000 AU troops are deployed in Darfur on a limited mandate
The tragedy of Darfur began to unfold in February 2003, when rebels rose up against the government, claiming the region had been neglected by Khartoum.
But the rebels failed to maintain their unity of purpose.
Their movement gradually fragmented. Today there are about 17 different groupings claiming to fight for the people of Darfur.
Whichever group it was, the fighting comes at a particularly unfortunate moment.
Discussions are due to begin in Ethiopia between the African Union and the United Nations.
These are designed to pave the way for peace talks between the government and the rebels in Libya on 27 October.
Diplomats believe that the current incident may be a show of strength by some rebels prior to those talks.
They point out that much the same thing took place before the last peace talks in the Nigerian capital, Abuja in May last year.
But with a number of rebel movements now refusing to attend next month's peace talks in Libya and the current upsurge of fighting, the prospects for reaching an agreement are beginning to look bleak.