On Wednesday Muhammad Ahmad Tugod, the chief negotiator for Sudanese group Justice and Equality Movement (JEM), said: "Negotiations have already collapsed because there are differences, strong differences between us and the Sudanese Government."
The African Union-mediated talks in the Nigerian capital Abuja are an attempt to end a conflict which erupted in February 2003.
"The AU is now suggesting suspending the talks for four weeks, but for us it as if the talks have collapsed," Tugod said.
Asked about an aid plan on which the rebels had been due to make their position known, Tugod said that Nigerian "President (Olusegun) Obasanjo is asking us to sign the humanitarian protocol, but it is a nonsense without the security agreement to sign the humanitarian protocol, so there is no reason for us ... "
More than 500,000 people have
been displaced by the fighting
"At this stage, we are just waiting for the last communique from the AU," he added.
"These are the facts: there is no progress, no agreement on the security issue after three weeks. That is why we will not sign the humanitarian protocol."
International pressure is mounting on Sudan to break the stalemate. Without an agreement Khartoum could find itself subject to international sanctions.
Sudanese Foreign Minister Mustafa Usman Ismail on Wednesday rejected a revised UN Security Council draft resolution threatening to consider sanctions on Sudan.
"This is imbalanced, unfair and we are rejecting it as we rejected the first draft," Ismail told a Cairo news conference.
US has concluded that genocide
has been committed in Darfur
The EU has been more cautious in its approach.
Earlier this week Dutch Foreign Minister Bernard Bot, holding the rotating six-month presidency, said the bloc would impose sanctions if Sudan did not take tangible measures to disarm the Janjawid.
The Janjawid are Arabic-speaking nomads accused of aiding the government in Khartoum to drive the sedentary Darfurian African tribes from their lands in western Sudan.
US Secretary of State Colin Powell told a Senate hearing on 9 September that evidence compiled by the United States led to the conclusion that genocide had been committed in Darfur.
He said the government of Sudan and the Janjawid bore responsibility and that "genocide may still be occurring".
The European Parliament is also set to call violence in Darfur "tantamount to genocide" later this week, echoing US statements and strengthening the EU stance.
The United Nations described the food and refugee problem created by the conflict as the world's worst current humanitarian crisis.