The rebel group was not identified, nor was the number of casualties released.
A spokesman for the African Union, which is monitoring a fragile ceasefire between government-sponsored militias and Darfur rebels, said it had "no evidence" to support the allegations.
"An investigation is still going on," added Noureddine Mezni.
The UN Security Council passed a resolution last week demanding that the International Criminal Court prosecute those behind Khartoum's two-year-old scorched earth campaign against ethnic minority rebels in Darfur.
Marked by mass killings, torture, rape and pillage, the campaign has left more than 300,000 dead and 2.4 million people displaced, according to a British parliament report.