The talks in Libya are going on in the absence of a number of key rebel groups, casting doubt on whether the negotiations can produce any meaningful outcome.
"We announce a ceasefire from this moment, and we will respect it unilaterally," Sudanese presidential adviser Nafie Ali Nafie said.
But one rebel leader voiced doubts about the move, saying it had failed to honour past such undertakings.
"The Government has already said several times since 2004 that they observed a ceasefire. They again spoke like this today," rebel leader Ahmed Ibrahim Diraige of the Sudan Federal Democratic Alliance said.
Mediators acknowledged the meeting was weakened by the absence of key rebel leaders.
On the eve of the African Union-United Nations talks, two main rebel groups, the Justice and Equality Movement (JEM) and the Sudan Liberation Army Unity faction, said they would not attend.
That decision emerged after another rebel chief, Abdel Wahed Mohamed el-Nur, founder of a third group, the Sudan Liberation Army (SLA), said he would not attend the talks.
The talks are the first attempt to gather Darfur rebels and the government around a negotiating table since 2006 when the African Union mediated Darfur peace talks in Abuja, Nigeria.
Libyan leader Moamar Gaddafi says peace talks will not succeed as key rebel groups have refused to attend.
Colonel Gaddafi, who is hosting the meeting in the Libyan city of Sirte, says there are limits to the usefulness of international intervention in what he calls a war between tribes.
Yahia Bolad, a spokesman for one of the rebel factions that is staying away from the summit, says the meeting is doomed to fail.
"It cannot succeed because it is empty talks," he said.
"We need UN troops to redeploy on the ground firstly because killing is going on and United Nations Security Council resolutions on Darfur remain ineffective which gives green light to Khartoum Government to continue its military operations." © 2007 Australian Broadcasting Corporation
