Talks to end the unbridled violence that has killed tens of thousands of people in Sudan's western Darfur (search) region collapsed Saturday with two rebel groups charging the government had not kept its end of the bargain.

Mediators worked late into the night trying to save the negotiations, which began Thursday at the African Union headquarters in the Ethiopian capital.

But the rebels, insisting the government fulfill a list of previous commitments first, walked out Saturday without having met the Sudanese government delegation.

"These talks are now finished," Ahmed Hussain Adam said on behalf of his Justice and Equality Movement and the Sudanese Liberation Army (search). "We are leaving Addis Ababa."

Ibrahim Ahmed Ibrahim, spokesman for the government delegation, said Sudan was not prepared to accept preconditions.

"The demands of the rebels are not acceptable and it is a disrespect to the Africa Union," Ibrahim said. "It is a delaying tactic ... The rebels are not serious."

But Sudanese Foreign Minister Mustafa Osman Ismail (search) said the government remains open to further negotiations.

"This round will not be the last one," he told reporters in the Sudanese capital, Khartoum.

African Union mediators were working to bring both sides back to the table. "Nobody told us the negotiations have ended," AU spokesman Adam Thiam said.

The rebels' main demand was an internationally supervised timeline for Sudan to make good on its promise to disarm shadowy Arab militias accused of killing tens of thousands of black Africans and driving more than a million from their homes in a systematic campaign of terror.

The insurgents also were seeking government commitments to respect previous agreements, allow an international inquiry into the killings, prosecute those responsible, lift restrictions on humanitarian workers and release prisoners of war. Finally, the insurgents wanted a more neutral venue for future negotiations, arguing that Ethiopia has close ties with Sudan.

Most of the rebels' demands were contained in a widely ignored cease-fire deal signed April 8 with the government.

"There's no progress being made because the government has refused these demands," Adam said.

He said government-backed attacks continued as recently as Thursday, when militia fighters known as the Janjaweed raided the southern Darfur village of Majreia, killing 17 people. His claim could not be independently verified.

Sudan also signed an agreement with U.N. Secretary-General Kofi Annan on July 3 calling for disarming the Janjaweed, deploying soldiers, facilitating aid and allowing international troops and monitors into Darfur.

The Janjaweed, the Arab militia that has torched hundreds of villages in a scorched earth policy that some human rights groups have called ethnic cleansing, did not attend the negotiations.

The rebels and refugees from Darfur have accused the government of arming and providing air support to the Janjaweed. The government denies any involvement in the militia attacks.

Sudanese Foreign Minister Mustafa Osman Ismail said Thursday in Sudan's capital, Khartoum, that his government needed more time to implement its commitments in Darfur, a vast and remote region two times the size of Idaho.

Nomadic Arab tribes have long been in conflict with their African farming neighbors over Darfur's dwindling water and usable land. The tensions exploded into violence in February 2003 when the two African rebel groups took up arms over what they regard as unjust treatment by the government in their struggle with Arab countrymen.

The United Nations estimates as many as 30,000 people have been killed in Darfur, but some analysts put the figure much higher. The death toll could surge to more than 350,000 if aid doesn't reach more than 2 million people soon, the U.S. Agency for International Development has warned.

Fighting in Darfur has continued even as peace negotiations advanced in a separate 21-year civil war in which rebels from Sudan's mainly animist and Christian south took up arms against the predominantly Arab and Muslim north. Both sides in Darfur are Muslim.

International pressure has mounted on Sudan to end the slaughter since April, when Annan warned that a new genocide could unfold in Sudan as the world marked 10 years since the 1994 slaughter that killed at least 500,000 people in Rwanda, most of them from the Tutsi minority.

The latest peace initiative followed a concerted diplomatic push by Annan and U.S. Secretary of State Colin Powell, who visited the region earlier this month.

Powell said Friday that he expects to hear from U.S. experts next week on whether Sudan officials should be charged with genocide.