Gunmen in the Democratic Republic of Congo have shot dead a United Nations peacekeeper following days of clashes between rival Congolese army factions.
UN peacekeepers are struggling to end the fighting in Bukavu
A UN spokesman said another UN peacekeeper was missing after the attack near the eastern town of Bukavu.
The unrest has caused about 2,000 people to flee to neighbouring Rwanda in recent days.
On Friday a UN helicopter gunship fired a rocket at supporters of former rebel commander Col Jules Mutebusi.
The UN accused Col Mutebusi's troops of looting and raping civilians. It warned that the rebel soldiers will be forcibly disarmed if they do not return to their barracks.
Saturday's attack on the UN peacekeepers happened at dawn as the military observers were sleeping, US spokesman Sebastian Lapierre told the Associated Press.
"The circumstances are not yet clear," he said.
"We don't know yet if the other observer is living or if he fled or was abducted during the attack."
Two other UN observers were in the UN camp during the attack, one of whom was lightly wounded by a bullet, Mr Lapierre said. Both have been evacuated from the area.
The UN has been trying to end days of clashes involving regular troops and rebel soldiers.
Under a peace deal agreed last year, all the Congolese warring factions were supposed to unite as a single army.
Former rebels are being integrated into a new national army
But violence between the two sides broke out on Wednesday evening. There are contradictory reports over who started the fighting.
The BBC's Robert Walker in Bukavu says the latest clashes underline the continuing difficulties in integrating former rebels into a unified national army.
Col Mutebusi and most of his troops are from the Banyamulenge ethnic group.
Most of the refugees who have arrived at a camp near the town of Cyangugu in Rwanda are also Banyamulenge.