Delegations gathered in Libya on Saturday to launch talks to end 4-1/2 years of conflict in Sudan's Darfur region but the absence of key rebels cast doubt on whether negotiations could produce any meaningful deal.

On the eve of the African Union-United Nations-mediated talks in Sirte, two main rebel groups -- the Justice and Equality Movement (JEM) and the Sudan Liberation Army Unity faction -- said they would not attend.

That decision came after another rebel leader, Abdel Wahed Mohamed el-Nur, founder of a third group, the Sudan Liberation Army (SLA), said he would not travel to Libya for the talks.

JEM-SLA Unity represent the biggest military threat to the Sudanese government and Nur has the most popular support among Darfuris. Analysts say without their representation in Libya peace talks had little chance of success.

U.N. and AU officials were playing down the prospect of a conclusive deal in Sirte, saying they hoped for some solid progress by year-end.

"It's not a question of one meeting or two," said AU spokesman Noureddine Mezni. "It's a long process. We hope that, as it evolves, others will join in."

Sudanese Minister of State for Foreign Relations Al-Samani Al-Wasiyla told reporters in Sirte: "We have a strong desire to reach a solution. They (rebels) should come to make their demands known to the government. Their demands should be objective and reasonable."

"We come here with an open heart to listen to them."

The talks are the first attempt to gather Darfur rebels and the government around a negotiating table since 2006 when the African Union mediated Darfur peace talks in Abuja, Nigeria.

Signed by only one rebel faction, the resulting deal had little support among the 2 million Darfuris trapped in displacement camps.

Rather than bring peace, it triggered fresh violence, as rebels split into more than a dozen factions, some preying on civilians, 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.

"SERIOUS SETBACK"

Analysts have warned that without full rebel representation the Libya talks would go the way of the Abuja deal.

JEM chief negotiator Ahmed Tugod Lissan told Reuters on Friday his group and SLA Unity decided not to go to Sirte because mediators had not invited the "genuine parties that should be part of the peace process".

He said organisers sided with the Sudanese government by inviting people specified by Khartoum.

Rebel leaders in Libya said those who stayed away were the founders of groups that had split and were now angry at seeing their former deputies taking part in the talks as their equals.

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.

Rebel delegates said up to seven factions would take part in the talks. A large Sudanese government delegation and representatives from the United States, the Arab League, China, Eritrea, Britain and Russia are also present, among others.

Opening speeches were expected on Saturday afternoon and rebel groups were due to meet to agree a common approach before signing a letter affirming their commitment to the talks.

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 year-end but, without a deal, some nations might be loath to commit troops.

Nigeria Libya Sudan