UN chief Ban Ki-moon has called for the urgent deployment of a joint force to halt violence in Sudan's Darfur region.
Aid agencies say the situation in Darfur is worse than ever
He told heads of state at the African Union summit in Ethiopia that they must work with the world body to end the deadlock over agreeing a UN-AU force.
The UN chief is to meet Sudanese President Omar al-Bashir shortly for talks on the issue.
Mr Bashir, who opposes UN peacekeepers, is due to take over the presidency of the AU in a move causing controversy.
Mr Bashir withdrew from the role last year because of the Darfur crisis.
Over the last few days, the Sudanese delegation has made it clear it still wants to take the role but this is now looking less and less likely, says the BBC's Amber Henshaw in Addis Ababa.
She says there has been enormous pressure from other African countries, the international community and NGOs, who say the situation in Darfur is worse than ever.
Mr Ban called on the Darfur rebel groups who had not signed a peace deal with the Sudanese government to do so.
"Together we must work to end the violence and scorched-earth policies adopted by various parties, including militias, as well as the bombings which are still a terrifying feature of life in Darfur," Mr Ban said.
Mr Ban says he wants concrete commitments that Sudan will open the door to a joint UN-AU force, to replace the current overstretched AU troops.
But there is no agreement on how many soldiers should be sent or who should lead the mission.
The UN chief says he will seek "a firm commitment to action" in talks with Mr Bashir. The UN wants to deploy 22,000 soldiers.
Nearly four years of fighting in Darfur between rebels and pro-government militias has led to the deaths of more than 200,000 people, and forced more than two million people to flee their homes.
AU Chairman Alpha Oumar Konare appealed to the Sudanese government to stop attacks in Darfur, in his opening summit address.
On the eve of the AU summit, the six biggest aid agencies working in Darfur warned that the humanitarian operation there was on the verge of total breakdown.
Mr Bashir's hopes of taking the AU presidency look like being thwarted
The "enormous humanitarian response in Darfur will soon be paralysed unless African and global leaders at the AU summit take urgent action to end rising violence against civilians and aid workers," they said in a joint statement.
The agencies - Action Against Hunger, Care International, Oxfam International, Norwegian Refugee Council, World Vision and Save the Children - said the conflict was now worse than ever, with aid workers being increasingly targeted as they seek to help threatened Darfuris.
Lobby group Amnesty International said Sudan's assumption of the AU chairmanship, while the AU was trying to mediate in Darfur, would be "a glaring conflict of interest", and would damage the organisation's credibility.
Chad, which borders Darfur, says it will leave the AU if Sudan takes over, while one of the Darfuri rebel groups said it would consider AU peacekeepers as enemies.
Instability in Somalia is also a prominent issue at the summit.
Ethiopia itself played a major role in ousting the Islamist forces that had taken control of much of southern Somalia and supports the interim government.
Ethiopia has begun cutting its force levels in Somalia, making a proposed AU peacekeeping force for Somalia an even more pressing issue, correspondents say.
Mr Konare told African leaders that only 4,000 out of the proposed 8,000-strong peacekeeping force to be deployed there had been pledged so far.
On Sunday, at least two people were killed in the Somali capital, Mogadishu, during an hour-long gunbattle between the police force and a local militia.
Officially, the AU agenda on Monday will be headed by climate change and scientific development, but BBC East Africa correspondent Adam Mynott says it is the other matters which are concentrating minds.