Sudanese government forces attacked rebel bases in Darfur on Friday, despite a ceasefire deal worked out in May.
Spokesmen for the rebels and the African Union say the government used air strikes and the Janjaweed militia to attack bases in KulKul and Jebel Moon. Both are in West Darfur.
A government spokesman confirmed the attacks, but denied using planes or the Janjaweed.
The government signed a cease-fire agreement with one of three rebel factions, the Sudan Liberation Army, in May. Friday's attacks were on bases belonging to factions that did not sign the agreement.
The African Union and United Nations envoys to Sudan issued a joint statement expressing "deep concern" about the violence because it violated the terms of the ceasefire, even if the attacks were on non-signatories. They also said they were particularly concerned about the fate of civilians in Jebel Moon.
Some information for this report provided by AFP and Reuters.