Also Thursday, UN Secretary General Kofi Annan lamented the lack of progress in talks aimed at bringing peace to the region where up to 300,000 people have been killed and 2.5 million displaced since pro- and anti-government forces started fighting in February 2003.
"Unless reversed, the slow implosion of the rebel movements threatens to extend the tragic situation in Darfur indefinitely," the Brussels-based International Crisis Group (ICG) warned.
ICG urged the international community to "better coordinate their messages to prevent the rebel movements from playing external actors against each other."
Darfur's main rebel group, the Sudan Liberation Movement (SLM), is split into two factions, one led by Secretary General Mani Arko Minawi and the other by Chairman Abdul Wahid Mohammed Nur. Each insists it controls the majority of the SLM's armed militants.
Relations between SLM and another rebel group, the Justice and Equality Movement (JEM) -- which is militarily inferior -- soured early this year, sparking open fighting.
"Rather than ending the rebellion, splintering of the SLA and JEM would likely lead to the prevalence of warlordism throughout Darfur and make a political solution to the crisis impossible," ICG said.
"The SLA... is increasingly an obstacle to peace. Internal divisions, particularly among its political leadership, attacks against humanitarian convoys, and armed clashes with JEM, have undermined the peace talks and raised questions about its legitimacy," ICG said in a report entitled "Unifying Darfur's Rebels: A prerequisite for Peace."
The report warned that with further rebels rifts, "there is little chance for real success at the African Union (AU) sponsored talks in Abuja, since the government is likely to exploit and exacerbate rebel weaknesses at the negotiating table."
Meanwhile in Geneva, Annan said he was "concerned about the slowness or the lack of progress in the Abuja peace process and we are going to try and apply as much pressure as we can on both parties, on the governement and on the rebels, because they both have a role to play."
"The only way to resolve this issue is through a political settlement," he told the executive committee of the UN High Commission for Refugees in Geneva.
"There can be no comprehensive peace in Sudan as long as the situation in Darfur is not resolved."
The huge country is in the delicate early stages of a marriage between the Islamist, Arab government in Khartoum and leaders of a separate rebel group that rose up in south Sudan in the early 1980s.
Last week, the AU, which is tasked with monitoring a ceasefire between Darfur's warring parties, implicated government forces in attacks on camps for displaced people in western Darfur, a charge Khartoum strongly denied.
The United States has on several occasions accused pro-government forces in Darfur of committing genocide.
ICG lamented that some of the Khartoum officials responsible for the bloodshed in Darfur had survived into a new power-sharing government, the fruit of peace talks with the southern Sudan People's Liberation Army rebel movement.
"The architects of the ethnic cleansing (in Darfur) have retained significant power in the new government of national unity, which thus far remains unwilling to take the military and political steps needed to resolve the conflict: neutralising the Janjaweed militias and establishing genuine power and wealth sharing between Darfur and Khartoum," the policy group warned.
Khartoum and the Darfur rebels signed a truce deal in April 2004, but this has been violated by both sides, worsening the dire humanitarian situation.
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