Two African Union peacekeepers from Nigeria and two civilian contractors have been killed in an ambush in Sudan's Darfur region.
It is not clear who carried out the attack on the peacekeepers
Three other soldiers were wounded in Saturday's attack in southern Darfur.
They are the first deaths among some 6,000 AU troops deployed in Darfur to monitor a truce between pro-government forces and non-Arab rebels.
A civil war in the region since 1993 has left tens of thousands of people dead and some two million homeless.
Last week, the AU accused Sudanese government forces of still carrying out what appeared to be coordinated attacks in Darfur alongside the Janjaweed militia - allegations denied by the Sudanese government.
Jean-Baptiste Natama, the acting head of the AU mission, said the attack had occurred while the peacekeepers were on patrol near Kourabishi in southern Darfur.
The two contractors were from a company that has been providing logistical support.
AU spokesman Noureddine Muzni told the AFP news agency that the three wounded soldiers were also Nigeria and were airlifted to an AU hospital in northern Darfur.
African Union efforts to get armoured personnel carriers which provide better protection have been obstructed by the Sudanese government, says the BBC's Jonah Fisher in Khartoum.
Mr Natama did not say who the AU suspected of carrying out the attack.
"We are investigating, but it is serious being the first time our personnel are killed in Sudan," he added.
EU foreign policy chief, Javier Solana, told reporters in Khartoum that the government was responsible for protecting AU troops in Darfur.
AU forces arrived in Darfur last year, and from an initial force of 500, the contingent has grown to about 6,200 with financial and logistical support from the European Union, the United States and others.
Peace talks between the Darfur rebels and the government in Khartoum have made little progress while violations have shaken the cease-fire in recent weeks.