By Andrew Heavens

KHARTOUM, Oct 12 (Reuters) - Armed militias ambushed a band of rebel fighters in south Darfur, sparking a gun battle that left at least 11 dead, insurgents said on Sunday.

The reports of fresh fighting in Darfur came at a sensitive time for Khartoum which has been stepping up diplomatic efforts to block a move by the International Criminal Court to indict Sudan's president for war crimes in the country's violent west.

Leaders from two rebel factions said the militias who struck late on Saturday were part of a build-up of fighters around the town of Muhajiriya, a past flashpoint in the Darfur conflict.

Rebels accused Sudan's government of backing the militias, saying Khartoum wanted to "spread chaos" in the region.

But aid sources in the area said it was unclear who was supporting the militia force.

The undermanned joint U.N./African Union UNAMID peacekeeping mission in Darfur confirmed fighting had taken place but said it was unclear who had taken part.

Law and order has collapsed in Darfur where more than five years of fighting has killed 200,000 and driven more than 2.5 million from their homes, say international experts. Khartoum puts the death toll at 10,000.

The fighting that started with an uprising of mainly non-Arab rebels in 2003 has fragmented into a conflict complicated by rival insurgent factions, armed bandits, government forces and tribal clashes.

Fighters from Darfur's rebel Sudan Liberation Army, and the smaller United Resistance Front, were ambushed as they travelled through open land east of Muhajiriya on Saturday afternoon, said URF leader Bahar Idriss Abu Garda.

"I lost one senior commander. Ten Janjaweed were killed," he told Reuters, using a commonly used name for government-backed militia in Darfur.

"It is very clear. The government has been reorganising the Janjaweed. They have been building up forces in the area. They want to create chaos in Darfur." He said the fighting ended as the attackers withdrew at sunset.

Earlier this week, the Aegis Trust put out a statement warning that Muhajiriya was under imminent threat of attack from a 300-strong Janjaweed militia that had been attacking villages in the surrounding area.

But an aid source told Reuters: "The only thing we can say for certain at the moment is that there is a militia in the area, that people have died and buildings in villages have been burned."

More than 45 people were killed during an attack on Muhajiriya in October last year. Sudan's government denied accusations they had been involved in the assault, despite reports that armed forces aircraft had bombed the settlement. (Editing by Charles Dick)

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