(Adds U.N. envoy's appeal in paragraph 6)

By Opheera McDoom

EL-FASHER, Sudan, Oct 26 (Reuters) - Darfur's two main rebel groups will not attend U.N.-African Union mediated peace talks in Libya, their leaders said on Friday, dashing any chance of an immediate deal to end 4-1/2 years of war in western Sudan.

"We decided not to go," Justice and Equality Movement (JEM) chief negotiator Ahmed Tugod Lissan told Reuters.

Lissan said the decision was made on Friday with the Sudan Liberation Army-Unity faction. SLA-Unity leader Abdallah Yehya confirmed the statement to Reuters in Darfur.

Lissan said mediators had not invited the "genuine parties that should be part of the peace process" to the talks set to begin in Libya on Saturday, siding instead with the Sudanese government by inviting people specified by Khartoum.

Mediators had hoped as many rebels as possible would come and negotiate a comprehensive ceasefire with Sudan's government.

"Time is on no one's side... This is a ticking bomb," U.N. envoy Jan Eliasson told reporters in Libya. "We hope these negotiations will include everybody."

Experts have warned that without full representation by key rebel leaders the Libya talks would go the way of a 2006 peace deal signed in Abuja, Nigeria.

Signed by only one rebel faction, that agreement had little support among the 2 million Darfuris trapped in displacement camps. Rather than bring peace, the deal triggered fresh violence, as rebels split into more than a dozen factions, some preying on aid workers and AU troops sent to the region to quell the violence but unable to protect themselves.

International experts say 200,000 people have died since rebels rose up against the government in 2003 charging it with neglect. The Sudanese government says the Western media exaggerates the crisis and only 9,000 people have died.

Darfur expert Alex de Waal called the withdrawal of JEM and SLA-Unity "a very serious setback".

"A (peace) process can be started but in the aftermath of Abuja it is essential that any agreement that is reached includes everybody. No process can be successful in the absence of JEM and SLA-Unity," de Waal said.

RAPE AND MURDER

Washington is becoming increasingly irritated with some of the players in Sudan and has threatened to impose new sanctions.

"We have been putting a lot of pressure on Sudan's government on a number of fronts but we are also watching them," a State Department official, who spoke in Washington on condition he was not named, said of the rebels.

A U.N. spokeswoman said Eliasson and AU envoy Salim Salim were both holding meetings in Libya ahead of the talks, including with representatives of various rebel groups.

Rebel delegates in Sirte said on Friday up to seven factions would take part in the talks and they pledged to persuade the missing rebels to join shortly.

But even before Friday's announcement, support was dwindling. Abdel Wahed Mohamed el-Nur, founder of the Sudan Liberation Army (SLA) rebel group, said he would not attend until a U.N. force was deployed to stop the rape, looting and murder that have characterised the conflict.

Nur, who commands few troops but has widespread support among Darfuris, said on Friday he also objected to Libya as the talks' venue because of Libyan leader Muammar Gaddafi's attitude toward the conflict and past efforts to "Arabise" Darfur.

Most recently, Gaddafi angered Darfuris by saying in a video conference the conflict in western Sudan was about camels and that those displaced to camps were enjoying the free food.

"I'm asking, if the international community really respected our people, why do they want to put us in a country which doesn't even recognise a human disaster and genocide against our people," Nur said from Paris, where he lives.

DO THEY POSTPONE?

Stephen Morrison, of the Center for Strategic and International Studies in Washington, said mediators had been trying to decide whether to postpone the talks.

"I think mostly what's going to happen is they'll try to get through this phase and put a longer time frame on it. (U.N. Secretary-General) Ban Ki-moon's push for these talks is twinned with getting some progress before some of the 26,000 forces deploy," Morrison added.

Khartoum agreed in July to allow a hybrid force of 26,000 U.N.-AU troops to deploy in Sudan to replace and absorb some 7,000 AU peacekeepers in western Sudan.

That deployment is expected to begin by the end of this year but Morrison said without a deal, some nations might be loath to commit troops. (Additional reporting by Lamine Ghanmi in Sirte, Libya, and Sue Pleming in Washington)

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