Two rebel groups and Sudan's government are attending fresh talks to try to end conflict in the Darfur region, which is in the grip of a humanitarian crisis.
More than a million people have fled their homes in Darfur
The discussions, sponsored by the African Union, are being held in the Nigerian capital, Abuja.
Previous talks collapsed in July when the rebels walked out after the government refused to meet their terms.
The meeting comes a week before a UN deadline expires for Sudan to quell the violence or face penalties.
Opening the meeting, Nigerian President Olusegun Obasanjo - current head of the African Union - said: "We are gathered here today to put our heads together, to rub
minds together because as far as we are concerned in Africa, part of one of our houses is on fire."
Meanwhile, UK Foreign Secretary Jack Straw is flying to Sudan to press the government to do more to end the violence.
He will meet ministers before travelling to a refugee camp in northern Darfur on Tuesday.
The UN estimates that up to 50,000 people have died in 18 months of violence blamed on Arab militiamen.
More than 1m displaced
Up to 50,000 killed
More at risk from disease and starvation
Arab militias accused of ethnic cleansing
Sudan blames rebels for starting conflict
More than a million people have been forced to flee their homes to escape the fighting.
Some 180,000 people are living in desperate conditions in camps on Chad's side of the border.
The international Red Cross says that later this week, it will begin its largest airborne operation since the 2003 war in Iraq to deliver aid supplies to Darfur.
Jimenez said the first flight carrying logistical supplies would leave Geneva for the Sudanese capital Khartoum on Tuesday evening.
Sudan denies backing the Arab Janjaweed militias and says the black African rebel groups - the Sudan Liberation Army and the Justice and Equality Movement - are responsible for the crisis.
The BBC's Anna Borzello in Abuja says the talks are being seen as a major test for the African Union and its ability to find African solutions to African problems.
Larger peacekeeping force
Speaking ahead of the meeting, Mr Obasanjo said in a television address that Sudan should do more to disarm the Janjaweed.
He added that the talks would focus on finding a political solution to the conflict by listening to what all sides had to say.
But he warned the discussions would be futile unless the participants were given the mandate to make and implement real decisions.
Our correspondent says the issue of sending a larger African Union peacekeeping force to Darfur is expected to be on the agenda.
Ahead of the talks, Sudan's top government negotiator, Mazjoub al-Khalifa, rejected such a move.
"I don't think there is a need for this," he said. "Simultaneously we will disarm the rebel
movements, the Janjaweed and other militia."
About 150 Rwandan soldiers are already in Darfur to help protect African Union ceasefire monitors. They will be joined by 150 Nigerian soldiers later this month.
Last month, a UN Security Council resolution gave Sudan 30 days to bring Arab militia under control or face international action.