Sudan's government and two main rebel groups from the Darfur region are due to meet for talks, but the start has reportedly been delayed to next week.
Hundreds of thousands have been driven from their homes
Fighting in Darfur in the west of the country has been going on for more than a year, after the rebels accused the government of neglecting the area.
Hundreds of thousands of civilians have been displaced, with more than 100,000 fleeing across the border into Chad.
A UN envoy has described Darfur as the world's worst humanitarian situation.
Diplomats in the Chadian capital Ndjamena - where the talks are to take place - told AFP news agency that representatives of the sides had failed to arrive on Friday.
It was now thought the talks might begin on Monday or Tuesday next week.
Push for peace
The United Nations humanitarian co-ordinator for Sudan, Mukesh Kapila, said that government-backed Arab militias have been systematically raping and killing in Darfur.
The BBC's Abaker Saleh in the Chadian capital says President Idriss Deby is to chair discussions which will include international observers.
This will be the fourth round of meetings seeking to secure a ceasefire.
The Sudan Liberation Movement (SLM) and the Justice and Equality Movement (Jem) rebels say the government has been oppressing blacks in favour of Arabs.
The fighting in the west of Sudan has intensified as government peace talks to resolve the 20-year war with southern rebels are nearing an end.
But the UN is concerned that this conflict could undermine the separate peace talks taking place in Kenya with the southern rebels.