(Adds Jamous comments)

KHARTOUM, July 31 (Reuters) - Eleven prominent international activists have sent an open letter to Sudanese President Omar Hassan al-Bashir requesting the release of Darfur rebel Suleiman Jamous, who some see as critical to Darfur peace efforts.

Jamous, the Sudan Liberation Army (SLA) humanitarian coordinator, has been virtually imprisoned for 13 months and is in urgent need of medical attention.

Jamous was the main liaison between the world's largest aid operation and rebels in Darfur, keeping looting of aid convoys down and humanitarian workers safe.

Last year the United Nations removed the elderly Jamous from Darfur to a U.N. hospital in neighbouring South Kordofan, without informing the government. Jamous needs a stomach biopsy which he cannot be performed in South Kordofan.

Khartoum calls him a "terrorist" and says he should be arrested.

The letter to Bashir requesting his freedom be guaranteed without fear of arrest was dated July 30.

Among the 11 signatories were South African Archbishop Desmond Tutu, former U.S. Ambassador to the United Nations Richard Holbrooke, former Czech President Vaclav Havel, and Nobel Peace Prize Laureate Jody Williams.

"In our different capacities, we have come to know and respect Suleiman for his humanitarian work in Darfur and his commitment to the well-being of the people of Sudan," the letter said.

"He exemplified the best tradition of civic activism in Sudan including personal piety and self-sacrifice in the cause of providing essential assistance to those in need."

Banditry and attacks have forced some aid agencies to leave parts of Darfur and others to reduce their operations.

A senior SLA Darfur rebel commander, Abdallah Yehia, has listed Jamous as a member of his delegation to attend a meeting in Tanzania organised by the United Nations and the African Union to prepare for peace talks with the Sudanese government.

Only one of three rebel negotiating groups signed a peace deal in Nigeria last year. Since the deal the non-signatories have split into more than a dozen factions.

"Suleiman Jamous is critical to the success of these talks and to rebel unity," U.S.-based Sudan expert and activist Eric Reeves told Reuters last week.

Suleiman said some field commanders had said they would not go to Tanzania, unless he was allowed to attend.

"I think ... I can influence these field commanders and politicians from the different factions to unite and pull them to the negotiating table," he told Reuters by telephone.

"I appeal to the government of Sudan and the rebels to reach a quick settlement to stop the suffering of the civilians."

U.N. envoy Jan Eliasson and his AU counterpart Salim Ahmed Salim hope to at least get a unified position from the fractured rebels in the Aug. 3-5 meeting in Arusha.

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