Oct 27 (Reuters) - Delegations from Darfur rebel groups, Sudanese government negotiators and international mediators gathered in Sirte, Libya on Saturday for peace talks aimed at ending 4-1/2 years of war in western Sudan.

The decision on the eve of the talks by two main Darfur rebel groups not to attend, however, cast doubt on the viability of the discussions.

Analysts say if the Libya talks lack support from key players any agreement coming out of them would likely suffer the same fate as a 2006 deal signed by only one Darfur faction. It did little, if anything, to end the violence.

Here are some details about the Darfur peace process and who may attend Saturday's talks:

* BACKGROUND:

-- Since the badly flawed 2006 Darfur Peace Agreement, signed by only one of three rebel negotiating groups, insurgents have split into more than a dozen factions, creating a chaotic security environment on the ground.

-- The first talks in Nigeria's capital Abuja were hindered not only by rebel divisions but by a disconnect between field commanders and the political leadership at the negotiating table.

-- The African Union-United Nations mediated talks in Sirte, Libya, to begin on Saturday, hoped to bring all those who have rejected last year's deal into the peace process.

* REBELS WHO WILL ATTEND PEACE TALKS:

-- JUSTICE AND EQUALITY MOVEMENT-COLLECTIVE LEADERSHIP: Led by Bahar Idriss Abu Garda, former vice chairman of Khalil Ibrahim's JEM. The group was formed in September after Ibrahim sacked Abu Garda and his ex-military chief Abdallah Abanda.

-- JEM-AZRAQ - Led by Idriss Ibrahim Azraq. Broke away from Khalil Ibrahim's JEM in 2007, accusing him of "ethnic chauvinism" and an inclination toward "Islamic fundamentalism".

-- REVOLUTIONARY DEMOCRATIC FORCES FRONT - Led by Salah Abu Surrah. An Arab-based Darfur rebel movement formed in 2006.

-- NATIONAL MOVEMENT FOR REFORM AND DEVELOPMENT - Led at the talks by Khalil Abdullah. It broke from JEM in 2004 and is said to have Chadian backing.

-- UNITED REVOLUTIONARY FORCE FRONT - Led at the talks by Alhadi Agabeldour. An Arab-led rebel group opposed to the government.

-- SUDAN LIBERATION MOVEMENT-G19 - Led at the talks by Khamees Abdullah. It is a small faction that rejected the 2006 Darfur agreement, and formed the military wing of the former National Redemption Front.

-- SUDAN FEDERAL DEMOCRATIC ALLIANCE - Led by Ahmed Ibrahim Diraige. The group, which rejected the Abuja peace deal, was part of the former National Redemption Front created in Eritrea in June 2006.

* WHO WON'T BE THERE:

-- JUSTICE AND EQUALITY MOVEMENT (JEM): Led by Khalil Ibrahim, one of the original negotiators, Ibrahim is also from the Zaghawa tribe. New weapons and a loose military alliance with SLA Unity has turned JEM into the largest military threat to the Khartoum government on the ground in recent months. Some distrust Ibrahim who they say has an Islamist agenda and has his eye on power in Khartoum rather than the rights of Darfuris. JEM has expanded its insurgency to the Kordofan region bordering Darfur. JEM said it would not attend the Libya talks, objecting to the presence of rebel groups they say had no constituency and no place at the table.

-- SLA/M (ABDEL WAHED): Led by popular SLA/M founder and its original Chairman Abdel Wahed Mohamed el-Nur, this group has few fighters mostly in western Jabel Marra, the central mountainous Darfur region where the revolt began. But Nur enjoys mass support among his Fur tribe, Darfur's largest, and in the hundreds of camps for Darfuri displaced. Nur, a lawyer, is based in Paris and has refused to attend until a force is deployed to stem the Darfur violence, which has recently surged anew.

-- SLA/M UNITY: The largest faction of fighters on the ground, this is a loose alliance of many major figures among Darfur's rebels. Officially led by Abdallah Yehya, it includes other prominent figures -- Sherif Harir, Abu Bakr Kadu and Ahmed Kubur. SLA-UNITY joined JEM in rejecting the talks, saying it objected to the presence of rebel groups they say had no constituency and no place at the table.

-- AHMED ABDEL SHAFIE: A Fur, he split from Abdel Wahed Mohamed el-Nur and also enjoys some support from the massive Fur tribe. Shafie and five other smaller factions have announced at a meeting in Juba they would also not attend, saying AU and U.N. mediators had not heeded rebel requests for a delay to allow them to form a united position and agree on a delegation. (Sources - Reuters, United Nations, International Crisis Group)

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