An army spokesman said: "Some of the rebels attacked the AU in Haskanita."
But Abdel Aziz el-Nur Ashr, commander of the armed opposition group Justice and Equality Movement (JEM), told Reuters news agency that they had moved their troops out of Haskanita four days earlier for other operations and blamed the attack on the government.
"There was an aggression from Sudan Armed Forces (SAF) from three directions on Haskanita and that attack was from the SAF.
"We don't have troops there."
Haskanita, in the far southeast of Darfur, has seen government bombardment and heavy fighting between the army, militias and rebels.
An alliance between JEM and the Sudan Liberation Army (SLA) Unity faction have become the largest military threat to Khartoum in recent months.
But there is also a volatile mix of bandits, tribal clashes and militia splinter groups threatening any attempt to achieve peace and stability in western Sudan.
"Genocide is a crime against humanity as a whole, not just against it's immediate targets. It therefore falls on the world at large to act."
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Mezni declined to speculate on who carried out the attack or elaborate on the nationalities of those killed.
Speaking from Addis Ababa, the Ethiopian capital, Said Djinnit, the AU commissioner for peace and security, told Al Jazeera: "We have some idea [as to which could be responsible] but we are still in the process of investigating to be able to exactly [identify] which group.
"We suspect that these are non-signatory movements, some of the factions which have been operating in that area."
Djinnit said: "We have always called for enhanced capacity for the armies, and we are in the process of deploying Unamid (African Union/Union Nations hybrid operation in Darfur), which is expected to have greater and enhanced strength and capacity."
Al Jazeera's John Terret, speaking from New York, said: "The mood at the moment at the UN is one of determination to keep their heads down and just continue planning, moving forward towards the peace conference in Libya and eventual deployment of the full 26,000 (Unamid force) complement.
"The attack has come at a very bad time, when it could easily have knocked confidence, but the mood here in New York is that it won't, that the conference will go ahead and that the boots will finally get on the ground."