Talks have been taking place in Libya for a second day to try to end the four-year war in Sudan's Darfur region.
The Sudanese delegation announced a unilateral ceasefire
As talks opened on Saturday, the Sudanese government announced a unilateral ceasefire.
But with key rebel leaders boycotting the talks, pessimism is growing that they will have any lasting impact, says the BBC's Jonah Fisher at the talks.
British PM Gordon Brown has called for the UN to speed up the deployment of its joint force with the African Union.
"If we can find a way forward with both a ceasefire and talks involving all the parties then there is hope for the area," he said.
"I call on all the parties to attend the ceasefire talks and peace talks."
At the gathering in the Libyan city of Sirte, Sudanese presidential adviser Nafie Ali Nafie said: "We announce a ceasefire from this moment, and we will respect it unilaterally."
However, several ceasefires have previously been agreed and none has ended the violence.
Some 200,000 people have died and two million have been displaced in the conflict.
Both the SLA-Unity and the Justice and Equality Movement (Jem) groups decided not to attend after the UN and the AU - who are mediating the talks - invited other rebel groups they claim have little support, said Mohammed Bahr Hamdeen, a senior Jem leader.
However, Libyan leader Muammar Gaddafi, who is hosting the talks, said that without the two groups' leaders present in Sirte, "we cannot achieve peace".
"As long as they object to this conference, then there is no justification for the international community to intervene," he said.
Mr Gaddafi also cast doubt on what the international community could achieve in Darfur.
"To internationalise a tribal problem is an exercise in futility," he said in his opening remarks to the gathering.
Fighting in Darfur began in 2003 when rebels attacked government targets.
Sudan's government then launched a military and police campaign in Darfur.
A 2006 peace deal faltered because it was signed by the Sudanese government and only one rebel group.
The rebel movement then splintered into at least 12 groups and sub-factions.
Large numbers of Darfuris are strongly opposed to the talks and doubt that the negotiations will lead to any concrete improvements on the ground, says the BBC's Amber Henshaw, in Darfur.
On Saturday, the UK announced an additional £6m of funds to support peace efforts in Darfur.
The funding will be divided between support for the AU-UN peace talks in Libya, which will receive £1m, and the Darfur Community Peace and Stability Fund, launched at the start of talks by the UN and international partners, which will receive £5m.
International Development Secretary Douglas Alexander said the money would be "used for grassroots development activity such as rehabilitating water points, health clinics and schools, to make an immediate improvement to people's lives and help to restore peace and confidence".