A rebel attack that killed at least 10 African Union troops at one of their army bases in Sudan's Darfur region has sparked international condemnation.
About 40 AU soldiers are missing and seven were badly injured in the raid, by heavily-armed men in 30 vehicles.
UN Secretary General Ban Ki-moon said the killings, which were blamed on splinter factions, were "outrageous".
Meanwhile, a delegation of notable world figures led by Archbishop Desmond Tutu has arrived in Sudan for talks.
The archbishop is leading a delegation of "elders" that includes former UN envoy to Iraq Lakhdar Brahimi, former US President Jimmy Carter, and Graca Machel, a children's rights advocate and the wife of Nelson Mandela.
The group is scheduled to meet Sudan's President Omar al-Bashir and opposition leaders in Khartoum on Monday before travelling on to refugee camps in Darfur.
The Saturday evening attack on the African Union military observers was the worst on its troops since their deployment to the region three years ago.
At the UN, Mr Ban's office issued a statement condemning the attack "in the strongest possible terms" and demanding those responsible "be held fully accountable".
The AU says 40 of the mostly Nigerian troops are still missing. Seventeen were found on Sunday in a nearby village.
Next year the UN and AU will jointly deploy the world's largest peacekeeping force - 26,000 troops - to Darfur.
It will absorb the current force of 7,000 African Union troops, which has been struggling to protect civilians in the region.
The BBC's Africa analyst, Martin Plaut, says the fighting comes at a particularly unfortunate moment, with discussions about to take place between the AU and UN to pave the way for peace talks between government and rebels.
Prospects of an agreement at the talks are starting to look bleak, he says.
AU-UN Joint Special Representative Rodolphe Adada labelled the weekend raid a "wanton and unprovoked act... that breaks every convention and norm of international peacekeeping".
A spokesman for the rebel Justice and Equality Movement (JEM) also condemned the attack, which he said was carried out by dissident commanders.
"They're looking for equipment - vehicles and weapons ... They found the AU an easy target," Ibrahim Jalil said.
The BBC's Orla Guerin, who visited the camp within rebel territory last month, said it was clear then that rebel fighters were too close for comfort.
Mission to Sudan
The attack provided a grim backdrop to the arrival of the group of self-styled "Elders" in the region.
The group came together at Nelson Mandela's invitation to find ways to tackle some of the world's toughest problems, such as HIV/Aids, poverty and conflict.
Commenting on the raid, Mr Tutu said: "It just shows how desperate the situation is, how desperately we've got to find a peaceful solution so that incidents of that kind don't happen."
The delegation will meet Mr Bashir and others in Khartoum before travelling to Juba, capital of southern Sudan.
They will then travel to Darfur where they will meet community leaders and displaced people living in camps.
At least 200,000 people have died in Darfur during a four-year conflict and more than two million have been forced from their homes.