Arab groups in Sudan's Darfur region could start their own rebellion, a leading think-tank has warned.
Sudan denies arming the Janjaweed militias
Some Arab communities felt they had been used by Sudan's government, says the International Crisis Group (ICG).
The ICG report also criticised the joint UN-African Union approach to Darfur, which led to last month's failed peace talks in Libya.
Pro-government Arab militias are accused of widespread atrocities against Darfur's black African people.
At least 200,000 people have died and two million made homeless since black African rebel groups took up arms in February 2002, complaining of marginalisation.
The ICG says over the past year, the fighting in Darfur "has mutated, the parties have splintered, and the confrontations have multiplied".
Darfur's rebel groups have broken up into numerous different factions - several of whom refused to attend the Libyan talks.
There have already been deadly clashes over land between different Arab groups in the region.
"Failure to respond appropriately would leave the international community an unwitting accomplice to the beginnings of Sudan's next war," said the ICG's Africa Research Director Daniela Kroslak.
The ICG said Sudan's ruling National Congress Party (NCP) was still fuelling the conflict.
"As part of its military strategy, the NCP has become the primary arms dealer in Darfur."
Sudan's NCP-dominated government has always denied arming the Arab Janjaweed militias. It says the West has exaggerated the scale of the problems in Darfur.
Some 20,000 UN peacekeepers are due to arrive in Darfur in the coming months, to boost the 7,000 African Union troops already there.
Sudan insists the UN troops should all be African, while the rebels have objected to peacekeepers from China - a close government ally.