Sudanese militias have burned civilians alive in the Darfur region, say African Union military observers.
Some one million people have fled their homes
Men rode into a village on horseback, looted the market and chained people up before setting them on fire, they say.
They are "believed to be Janjaweed" - the pro-government militias accused of ethnic cleansing against non-Arabs.
Correspondents say this criticism of pro-government forces is rare from African countries and will bolster moves for a UN resolution on Sudan.
The UN Security Council is set to consider imposing sanctions on Janjaweed leaders this week.
Sudan denies backing the Janjaweed and says that sanctions are not the answer to the crisis.
The African Union fact-finding team visited the village of Suleia to investigate reports of an attack on 3 July.
They say that the "entire Ehda village had been burnt and deserted, except for a few men" two days later.
Darfur is a vast region, the size of France, with very few roads, so it can take time for the observers to verify reports of violations of the ceasefire agreed in April between the government and two Darfur rebel groups.
"The CFC [ceasefire commission] concluded that this was an unwarranted and unprovoked attack on the civilian population by the Janjaweed (but) could not substantiate allegations that the Sudanese forces fought alongside the Janjaweed," their report said.
Sudan has repeatedly promised to disarm the Janjaweed.
An African Union (AU) statement said it is actively considering turning its military observer mission into a fully fledged peacekeeping force, with particular emphasis on disarming the Janjaweed.
It has previously said it would send 300 troops to Darfur to protect its observers in July but these have not yet been sent.
The UK has also raised the possibility of sending troops to Darfur to protect the one million people who have fled their homes.
But US Secretary of State Colin Powell said on Tuesday it was premature to speak of military intervention in Darfur.
Aid supplies are running low, aid workers say
Sudan's Foreign Minister Mustafa Osman Ismail also warned that Sudanese soldiers would fight back if foreign troops are sent to Darfur.
Aid agencies warn that thousands more could die in refugee camps from disease and starvation unless help arrives immediately.
Sudan summoned senior UK and German diplomats on Tuesday to protest after the EU pushed for sanctions over the Darfur conflict.
Mr Ismail also said Khartoum was disarming the Janjaweed militias - but said this could not be completed unless rebel groups also surrendered their weapons.
Sudan is lobbying to block the proposed UN sanctions - contained in a US draft resolution being discussed in a closed Security Council session this week.
China, Pakistan and Algeria are said to oppose immediate sanctions, but US diplomats say they are confident that a majority of UN Security Council members back their draft.
There is also some opposition to US efforts to push for the Council to vote on the issue this week.
The Arab League has told the UN Security Council to "avoid precipitate action" and give Sudan enough time to honour its pledges.
African leaders are set to seek an "African solution" to Darfur at a special summit in Ghana on Thursday, called by the AU chairman, President Olusegun Obasanjo of Nigeria.