Mediation talks in Nigeria ended on Sunday, just before the start of a UN deadline requiring Sudan to address the crisis or face serious political and economic consequences.
But negotiations appeared to end in deadlock after both rebel groups staged a 24 hour boycott - accusing Khartoum of killing 75 civilians since Thursday.
Ahmad Muhammad Tugod, negotiator for the Justice and Equality Movement (JEM) rebel group, told journalists in Abuja on Sunday: "There is a big distance between what we think about improving the humanitarian situation in the camps and what the government thinks."
Need for calm
The talks had aimed to find a political solution to the conflict, which has its origins in land disputes between sedentary peoples and nomadic tribesman over scant resources.
Rebels accuse the so-called nomadic Janjawid militias of mass killing and rape. An AU ceasefire committee has begun investigations into their allegations.
Over 1 million people have fled their homes in the arid western region, which is roughly the size of France, since the conflict started 18 months ago.
Many thousands are also believed to have died in the fighting.