By Michael Georgy

KHARTOUM, March 27 (Reuters) - Former Sudanese rebels said on Tuesday a peace agreement signed last year is in danger of collapse if the government rejects its demands following weekend clashes that left at least 10 people dead.

Eight members of the rebel group, the Sudan Liberation Movement (SLM), and two Sudanese police officers were killed in the clashes on Saturday in the city of Omdurman, on the west bank of the Nile opposite Sudan's capital.

The SLM was the only one of three Darfur rebel negotiating factions to sign the peace deal with Khartoum in 2006, but both sides are now in a tense standoff.

The group has demanded the release 93 of its members it said were arrested after the violence, the return of the bodies of those killed and the withdrawal of heavily armed government forces still surrounding its headquarters in Omdurman.

"It is obvious what will happen if our demands are not met," said SLM spokesman al-Tayyib Khamis. "This would endanger the peace deal. We may have to reconsider our position."

Special United Nations and African Union envoys for Darfur said the clashes threatened to undermine a wider peace deal with rejectionist rebels and called for an independent investigation.

"They don't help the peace process. They lower the morale of those concerned. They send the wrong messages," Salim Ahmed Salim of the African Union, which brokered the Darfur Peace Agreement, told a news conference.

Adding further to concerns, United Nations envoy Jan Eliasson said that tribal fighting was now killing more people than clashes between the government and rebels.

"I can't give you the exact figures (of deaths) but it's already in the hundreds this year. Some of it hasn't really surfaced much in the world media," he said.

"The tribal fighting is a sign of the deterioration in a situation we have to watch very carefully."

Experts estimate 200,000 people have been killed and 2.5 million driven from their homes in Darfur to miserable camps in four years of rape, killing and pillage. Washington calls the violence genocide, a term Khartoum rejects.

Rebels and residents say nomadic Janjaweed militias backed by the government carry out widespread abuses but the government calls them outlaws and says it has no links to the militias.

After the peace deal, SLM leader Minni Arcua Minnawi became senior assistant to the president with special responsibilities for Darfur. But he has complained the dominant National Congress Party (NCP) lacks political will to implement peace.

The fighting broke out on Saturday when police surrounded the SLM headquarters, the state-run Sudanese Media Centre said.

It said the SLM had refused to hand over members involved in a traffic accident two days earlier to authorities.

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