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By Opheera McDoom

KHARTOUM, Oct 11 (Reuters) - Achieving a credible ceasefire to stop the violence in the western Sudanese region of Darfur will be the priority at peace talks this month, U.N. envoy Jan Eliasson said on Thursday.

He urged all the fractured Darfur rebel factions to attend the talks, due to start in Libya on Oct. 27, saying they were "the moment of truth" to stop the violence in western Sudan that has created one of the world's worst humanitarian disasters.

"The first very concrete step ... is that we will go for and hopefully achieve a credible cessation of hostilities," Eliasson told reporters in Khartoum.

"I find the situation on the ground ... deeply alarming. The military escalation is a great source of concern."

He said invitations to most rebel groups would be sent for the first stage of talks.

But some rebel leaders have already said they will not go unless only one delegation from the Justice and Equality Movement (JEM) and one from the Sudan Liberation Army (SLA) talk with Khartoum.

But with more than a dozen factions emerging since last year's peace deal, signed by only one group, that is unlikely to happen.

Eliasson said preconditions were dangerous.

"To miss this opportunity is a tragedy -- this is the moment of truth," he said.

"Haven't we seen enough violence and hopelessness? Don't we see what is happening to the social fabric of Darfur ... torn apart completely -- is this what we want to perpetuate?"

Abdel Wahed Mohamed el-Nur, the SLA founder with huge popular support, said he would not attend the talks until a U.N. force was deployed to stem the violence, which has exploded with civilians, African Union peacekeepers, militias and rebels caught up in clashes in southern Darfur.

On Thursday, the only rebel faction to sign the 2006 deal called for an international investigation into an attack on their forces in Muhajiriya, where at least 45 were killed and dozens injured.

The SLA blames Sudan's army for the attack, although it has denied any involvement.

"There has to be an international investigation immediately," Minni Arcua Minnawi, the head of the SLA who became presidential adviser in Khartoum.

"We are committed to the peace and the ceasefire but we want the government to not repeat any action like this."

FORMAL COMPLAINT

The attack on Muhajiriya, Minnawi's main town, was the latest in an upsurge of violence in southern Darfur after the worst attack on African Union peacekeepers since they deployed, killing and wounding at least 20 and destroying their base in Haskanita.

Two aid agencies working in the town evacuated 29 staff after they were trapped by the fighting.

Minnawi said his movement had written a formal complaint to the United Nations and African Union about the assault.

The army blamed tribal clashes between the Zaghawa and Maaliya in the area. Minnawi's party accused Khartoum earlier this year of arming the Arab Maaliya tribe.

"This is the behaviour of the government. They will never commit to any agreement," Minnawi told Reuters from Darfur.

"This will have a negative effect on peace talks."

Minnawi is in Darfur to meet rebel factions who rejected the last deal to persuade them to attend the talks, mediated by the United Nations and the African Union, and unify their ranks.

In a report made public on Wednesday, U.N. Secretary-General Ban Ki-moon said he was "extremely concerned" about the "unacceptable" violence in the vast region, which he said was "not contributing to an atmosphere conducive to the peace talks".

He said the attack on the AU peacekeepers "confirms that the ... force which will be deployed to Darfur must be sufficiently robust to defend itself from spoilers and protect civilians from attack". A 26,000-strong AU-U.N. peacekeeping force is planned.

Mostly non-Arabs took up arms in early 2003 accusing Khartoum of neglect. International experts estimate 200,000 have died and 2.5 million driven from their homes in 4-1/2 years of fighting.

Khartoum puts the death toll at 9,000 and says the West has exaggerated the conflict in Darfur.

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