Sudanese officials have sent mixed signals about whether the government will stick to the deal, with some officials questioning the nature of the "hybrid" force.

"This is the moment of truth for Darfur," U.N. Undersecretary General for Humanitarian Affairs Jan Egeland told reporters in the Sudanese capital, Khartoum. "We are playing with a powder keg, it could get infinitely worse."

More than 200,000 people have been killed and 2.5 million displaced by three years of fighting in the vast, arid Darfur region of western Sudan. An ill-equipped African Union mission has achieved little in preventing the violence from worsening.

U.N. chief Kofi Annan and his AU counterpart Alpha Oumar Konare reached an agreement with Sudanese envoys Thursday for a mixed U.N.-AU force of some 20,000 peacekeepers.

Egeland, who said the ability for aid workers to conduct their mission was "crumbling" because of the violence, hoped the agreement reached in the Ethiopian capital of Addis Ababa would finally allow "an effective international force" to deploy.

But Sudan's Foreign Minister Lam Akol said his government had not committed to a mixed peacekeeping mission.

"What we have agreed upon is that the force should remain African and it be assisted by the United Nations," Akol told reporters Saturday. "There is no way the main fighting force would be a mixed one."

Other officials have stated the notion of a combined force was not an issue as long as the leadership and the strongest contingent remain African.

The mixed messages from Sudan's political leadership may be an effort not to appear to be backing down in the face of Western pressure after months of vehement opposition to any U.N. peacekeeping in Darfur.

Egeland said he was confident all parties involved would soon reach a final agreement.

"I have no reason to disbelieve the sincerity of the Sudanese negotiators in Addis," Egeland said. But he warned Darfurians were dying every day and hoped time would not be wasted "wrangling on words."

He said Darfur's crisis had worsened and that there were now more than 4 million people depending on international aid to survive.

A joint Sudanese army and janjaweed militia killed several civilians in the Jebel Marra mountain range late Friday, hours after he was barred from going there, Egeland said. He did not specify how many people had died in the rebel stronghold.

Egeland, who was on his last mission to Sudan before his term as U.N. humanitarian chief ends in December, said Sudanese authorities had barred him from four of the six destinations previously agreed upon. He said he was ending his trip early.

More than $1 billion in humanitarian aid is being spent on Darfur this year, he said, adding that the largest donor was the United States.

"The jury is really out now. Is the largest humanitarian effort on earth going to collapse or succeed?" he asked.