An attack on an African Union army base in the Sudanese region of Darfur has killed at least 10 peacekeepers.
Thirty vehicles overran the base, and 50 AU soldiers were missing and seven seriously injured. Vehicles and property were looted or vandalised.
Rebel sources told the BBC that the raiders were members of breakaway factions from two rebel groups.
The attack came as S African Archbishop Desmond Tutu arrived in Sudan bringing a new peace initiative for Darfur.
The casualties were the most serious suffered by the AU mission since it arrived in 2003, an AU statement said.
AU-UN Joint Special Representative Rodolphe Adada said he was profoundly shocked and appalled by the "outrageous and deliberate" attack, which happened on Saturday evening at a base in Haskanita town.
BBC 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.
A spokesman for the rebel Justice and Equality Movement (JEM) condemned the attack, saying it was carried out by three dissident commanders from his own movement, in conjunction with one of the groups that broke from the Sudan Liberation Army.
"It's a group which has been expelled," Ibrahim Jalil said.
"They're looking for equipment - vehicles and weapons. They couldn't get these within JEM, and they don't have the capability to fight government forces. They found the AU an easy target."
Sources told the BBC that the attackers made off with all the weapons and vehicles they were able to take, and burned the vehicles that remained.
The AU statement described the attackers as "a large and organised group of heavily armed men", but did not say whether they were rebels or government troops.
"It is staggering to imagine what could possibly have been the intentions of those who perpetrated this wanton and unprovoked act," Mr Adada said.
"Not only was it a flagrant violation of the ceasefire but an unconscionable crime that breaks every convention and norm of international peacekeeping."
Mission to Sudan
About 7,000 African Union troops are deployed in Darfur on a limited mandate.
The UN Security Council has approved a 26,000-strong peacekeeping force to expand the current AU force, which has been struggling to protect civilians.
On Sunday Archbishop Tutu arrived in Khartoum - in the latest initiative to bring peace to Darfur.
The archbishop is leading a delegation of "elders" that includes former UN envoy to Iraq Lakhdar Brahimi, US ex-President Jimmy Carter, and Graca Machel, a children's rights advocate and the wife of Nelson Mandela.
The group came together at Mr 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 attack on the AU, 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 BBC's Amber Henshaw in Khartoum says it is no coincidence that Darfur is the focus of the group's first mission.
At least 200,000 people have died and some 2m have been forced from their homes during the four-year conflict.
The delegation will meet President Omar al-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.