(Adds another faction going to Arusha)

By Opheera McDoom

KHARTOUM, Aug 2 (Reuters) - Commanders from a large Darfur rebel faction have threatened to boycott a crucial unity meeting in Arusha, Tanzania, on Friday unless the Sudanese government allows a senior rebel figure to attend.

The latest hitch in the peace process sponsored by the African Union and the United Nations coincided with the first African troop pledges to a new peacekeeping force, approved by the United Nations this week and given guarded backing by Sudan.

Expected to cost more than $2 billion in the first year, the "hybrid" force will assume authority by Dec. 31 over 7,000 AU soldiers who have struggled to stop the violence in Darfur.

International experts say 200,000 people have been killed and 2.5 million displaced in Sudan's western Darfur region since a rebellion erupted there in 2003. Sudan says 9,000 have died.

Five African nations -- Burkina Faso, Nigeria, Egypt, Cameroon and Ethiopia -- have pledged to send peacekeepers, a top AU official said on Thursday, and South Africa said it would consider adding to the 97 soldiers it already has there.

But the task of building up the force to its target strength of 26,000 is expected to take many more months.

The U.N. and the AU will chair the Arusha meeting, starting on Friday, with the aim of getting the rebels to agree on a joint negotiating platform for peace talks.

A faction leader from the Sudan Liberation Army (SLA) told Reuters its commanders would not attend the talks unless the SLA's humanitarian coordinator was allowed to go too.

Suleiman Jamous has been virtually imprisoned in Kordofan, near Darfur, for 13 months since being moved to a U.N. hospital there for treatment. Sudan says it will arrest him if he leaves.

Observers say Jamous is crucial as the rebels' link with the humanitarian operation in Darfur, the world's largest, which has been forced to scale down because of attacks on its convoys, leaving some 500,000 people out of reach of aid.

They also say he is key to uniting rebel factions and military commanders in Darfur with political leaders outside the region, whose differences are a major obstacle to a settlement.

INTERNATIONAL CALL

This week 11 prominent activists including South African Archbishop Desmond Tutu, former U.S. Ambassador to the United Nations Richard Holbrooke, former Czech President Vaclav Havel, and Nobel Peace Laureate Jody Williams, wrote to Sudanese President Omar Hassan al-Bashir requesting his release.

"Our commanders on the ground are saying that unless Jamous goes to Arusha, they will not go," SLA faction head Abdallah Yehia told Reuters.

"I told the U.N. and AU, but they say it is difficult. I ask why is it difficult? He is in a U.N. hospital," said Yehia, who is from the SLA-Unity group. Yehia said he would travel to Arusha but his commanders were refusing to go.

Sudan said on Wednesday it was ready to consider releasing Jamous but neither U.N. Darfur envoy Jan Eliasson nor his AU counterpart Salim Ahmed Salim had raised the issue in talks.

U.N. spokesman George Somerwill said the issue had been raised with the government, which had said it would consider his release once the peace talks began.

Key field commanders Jar el-Neby and Suleiman Marajan agreed on Thursday to attend the talks, improving the prospect of progress. "We have received an official invitation and we are prepared to go to Arusha," Jar el-Neby told Reuters from Darfur.

A visiting U.N. rights envoy called on Sudan to curb the powers of its security forces amid continuing rights violations across the country.

Sima Samar, the U.N. special rapporteur on human rights in Sudan, said during a visit to Sudan there were some positive signals from Khartoum but still many problems, including the arrests of political opponents and journalists.

Mostly non-Arab rebels took up arms in early 2003 accusing the central government of neglect. Khartoum mobilised mostly Arab militias, known locally as Janjaweed, to quell the revolt.

After an AU-mediated peace deal last year, signed by only one of three rebel negotiating factions, the non-signatories split into more than dozen factions, complicating peace moves. (Additional reporting by Tsegaye Tadesse in Addis Ababa and Paul Simao in Pretoria)

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