Jem leaders have been angered by other rebels holding separate talks
A row between Darfur rebel groups has thrown into doubt a peace deal between the rebels and Sudan's government.
One rebel group signed a preliminary deal last month, and a final agreement was due to be completed on 15 March.
But the BBC's correspondent in Khartoum says talks have stalled as another rebel faction pursues separate talks.
The framework deal agreed last month was hailed as a major breakthrough, prompting President Omar al-Bashir to claim that the war in Darfur was over.
The Justice and Equality Movement (Jem), seen as the main rebel group, signed up to the deal on 23 February.
The BBC's James Copnall in Khartoum says the government and the rebels were then supposed to agree on a number of difficult issues before signing the final agreement three weeks later.
The issues included integrating Jem's fighters into the Sudanese army, returning millions displaced by the conflict to their homes, and the finer points of power-sharing.
Our correspondent says talks on these issues have not really started.
He says another band of rebels have grouped together under the name Liberation and Justice Movement (LJM) and held separate negotiations with the government - much to the annoyance of Jem.
And fighting has continued between government forces and another rebel group, the Sudan Liberation Army Abdul-Wahid faction, according to several sources.
The government was keen to sign the deal before a national election due in April - seen as Sudan's first genuine multi-party poll in two decades.
Opposition groups and rebel factions have repeatedly called for the election to be postponed amid fighting in southern Sudan and instability in Darfur.
Some 300,000 people have died and 2.7 million forced from their homes since rebels took up in arms in Darfur in 2003, according to UN estimates.
Divisions between the numerous rebel factions in Darfur have long been seen as a major obstacle to bringing peace to the region.