The three-day conference following the recent north-south peace deal that ended two decades of civil war in Sudan, aims to help southern Sudan's fractious groups take advantage of billions of dollars in post-war reconstruction aid pledged by donors, and form a semi-autonomous government before August.
At the meeting on Monday, the Sudanese government vowed to rein in pro-Khartoum militias.
Sudan's Second Vice-President Moses Machar said Khartoum would put an end to militia activities in regions that it controls, notably eastern and western Upper Nile states in central Sudan.
But UN Secretary-General Kofi Annan said in a report released on Monday the Sudanese government was still battling rebel groups in the country's western Darfur region despite numerous agreements to stop the fighting.
The report said Darfur had seen little improvement in the last month.
According to Annan, while attacks on civilians declined, violence against international personnel rose, and the region saw more military activity by the government and its Janjawid allies, as well as by rebel groups.
"The killing of civilians and combatants alike must stop and a genuine ceasefire must be observed," Annan said.
The monthly report, delivered to the UN Security Council, said March saw a rise in "banditry, looting and hijacking of vehicles." Three attacks in particular were troubling, including one on 22 March that seriously wounded a US foreign aid worker, Annan said.
"The killing of civilians and combatants alike must stop and a genuine cease-fire must be observed"
UN Secretary-General Kofi Annan
In mid-March, the UN pulled staff from four western areas of the Darfur region because of threats against foreigners and other staff.
An estimated 180,000 people have died in the conflict since February 2003, when two rebel groups took up arms against the government to win more political and economic rights for Darfur's African tribes.
In the report, Annan warned of the possibility that Sudanese who believe they may be on the list of suspects handed to the International Criminal Court could resort to attacks against the UN and international aid groups.
Annan's report did note some positive steps: There were fewer attacks against civilians, while humanitarian assistance is reaching more people. Aid workers have been able to get to several remote areas as well, he said.
Tensions still high
Machar made his pledge to crack down on militias on the eve of the reconciliation conference in Nairobi.
He said: "If the problem is from the forces that are allied to the government, the Sudan government will have to take steps to stop it.
"If those militias have gone out of their way to break the ceasefire, it is time to show what the government of Sudan shall be doing," he said.
Garang has been accused of
Despite the peace deal and the promised aid, tensions remain high between the government, its proxy militias in the south and the Sudan People's Liberation Army (SPLA).
Many militias have refused to deal with SPLA leader John Garang whom they accuse of being dictatorial.
"We came with an open heart, our priority is peace," Machar said when asked about the prospects for the lingering resentment being overcome in Nairobi.
"You cannot deny the fact that when a people have been involved in a bitter struggle for the last 20 something years, it is not easy when the war stops that you easily come together and put things right," he said.
"We want to solve the problems," said Machar, who heads Khartoum's delegation.
SPLA officials have said they hope the meeting will be able to bridge differences between all sides in the south on the nitty-gritty of the peace deal, and devise ways of dividing among themselves oil revenue that will be shared with the north under a complex formula in the agreement.
"We came with an open heart, our priority is peace"
Sundan second vice-president
Sudan watchers say the conference offers an opportunity to completely pacify the region, the theatre of more than two decades of war between the south and the north.
"The conference offers an opportunity to southern political and armed groups to define a common ground of the implementation of a peace deal," said David Mozersky, a Sudan analyst with the International Crisis Group.
The conflict claimed at least 1.5 million lives and forced at least four million others to flee their homes.