The African Union says it is still trying to find out what prompted a large group of rebels to launch a deadly attack on its peacekeeping force in Sudan's western Darfur region on Saturday. At least 10 A.U. soldiers were killed and nearly 50 others are missing in the worst single attack since the peacekeeping mission began there three years ago. VOA Correspondent Alisha Ryu has the latest from our East Africa Bureau in Nairobi.

A spokesman for the Addis Ababa-based African Union, Geoffrey Mugumya says his organization is in deep mourning over the loss of at least seven peacekeepers from Nigeria, a police officer from Senegal, and two military observers from Botswana and Mali, who died after gunmen overran their base outside the town of Haskanita in south Darfur late Saturday.

"We are upset because this is the first time that we have lost 10 people at once since our mission began in Darfur," said Mugumya. "This was terrible for us. We are all devastated."

Mugumya declined to say who may have carried out what the African Union and the United Nations have called an outrageous and deliberate attack. Mugumya says the commander of the peacekeeping force is in Haskanita now and will soon issue a full report to the pan-African body.

But A.U. officers have told an Associated Press reporter at the scene that they believe the gunmen were fighters belonging to a faction of the Sudan Liberation Army, one of the main rebel groups in Darfur. The A.U. officers say the rebels first pounded the base with rocket-propelled grenades and mortars before attacking in armored vehicles.

Reports say about 150 peacekeepers inside the base fought a fierce battle with the rebels for several hours before the peacekeepers ran out of ammunition. The Sudanese army came to their rescue early Sunday, chasing away the rebels and evacuating the remaining soldiers from the destroyed base.

On Sunday, the commander of the rebel Justice and Equality Movement told the Reuters News Agency that he believed the attack was carried out by breakaway rebels from his group, working together with members of the Sudan Liberation Army Unity faction.

VOA was unable to reach the breakaway rebel leaders for comment.

The area around Haskanita was the scene of a large Sudanese government offensive against rebel fighters in recent weeks.

International experts estimate that as many as 200,000 people have died and more than 2.5 million people have been displaced since non-Arabs in Darfur took up arms against Khartoum's Arab-dominated government in 2003. The rebels accuse the government of marginalizing and neglecting the remote region for decades.

The United States accuses the Sudanese government of using proxy fighters called the Janjaweed to commit genocide against civilians in Darfur. Khartoum says about 9,000 people have died in the conflict.

U.N. and African Union-mediated peace talks aimed at negotiating an end to the conflict are scheduled to begin in Libya later this month. But senior rebel leaders have urged a delay, arguing that continuing violence in Darfur shows that no one is yet prepared to enter into a genuine political dialogue.