CAIRO, June 4 (Reuters) - A rebel group in western Sudan on Friday threatened to resume fighting against Sudanese government forces if Khartoum and a separate rebel group in the south do not include them in a peace agreement to be signed in August.

The Justice and Equality Movement (JEM), one of several rebel groups operating in the western region of Darfur, said that any agreement that left out Darfur would be incomplete.

The JEM and the rebel Sudan Liberation Movement have an uneasy truce with government forces in Darfur but JEM said on Friday that Arab militias linked to the government attacked and burned down several villages on Thursday, killing 10 people and stealing some 1,350 head of cattle.

A new third group, the National Movement for Reform and Development, also reported the militia attack and said that it reserved the right to retaliate. The National Movement is a splinter group from JEM, a JEM source said.

The Justice and Equality Movement said in a statement: "If the concerns of the other regions (of Sudan) are ignored this will leave the Sudanese Justice and Equality Movement with no option but to continue its armed struggle until victory."

It was referring to a peace deal between Khartoum and the rebel Sudan People's Liberation Movement (SPLM), who are signing a collection of "protocols" in the Kenyan capital Nairobi on Saturday as a prelude to reaching a comprehensive peace deal for the south, expected to be ready in August.

"The final agreement that is the sum of the protocols signed between the two sides is an incomplete agreement that does not encompass the remaining regions of Sudan," JEM said.

The agreement commits "gross injustice on the regions it does not include", the statement added.

The JEM and the Sudanese Liberation Movement signed a truce with the Sudanese government on April 8 to allow urgent aid to reach over a million people affected by the conflict. Since its signing both sides have traded accusations of violations.

An African Union observer mission arrived in the country on Thursday to monitor the ceasefire and investigate the allegations of violations.

Fighting has raged in Darfur since February 2003, adding to Sudan's woes as it tries to end the long war in the south.

The attack on Thursday was in the Jebel Moon area, 40 km (25 miles) southeast of Kulbus in West Darfur state, JEM said. It blamed the Janjaweed, the local name for Arab militias who have been attacking African villagers.

The rebels say the government helps the Janjaweed but the government says they are outlaws beyond its control. Sudanese officials were not available to comment.

(Reporting by Nima Elbagir, editing by Sami Aboudi; Cairo bureau:
+20-2578-3290/1)