The recent violence targeting UN personnel prompted the UN peacekeeping force (MONUC) to begin evacuating dozens of its staff from the Democratic Republic of Congo to neighbouring Republic of Congo.
The UN staff - civilian employees and military observers - who were evacuated on Sunday crossed the Congo river separating Kinshasa and Brazzaville on light boats hired by MONUC, along with a vessel loaded with MONUC vehicles, a UN official said.
Two UN trucks were attacked on Sunday in Rugari, about 40kms north of Goma, eastern Congo's main city.
The convoy had been dispatched to investigate reports of an attack on a civilian vehicle, said Jacqueline Chenard, the UN spokeswoman in Goma.
Insurgents from Rwanda
Residents told UN personnel that the ambush was carried out by insurgents from neighboring Rwanda based in eastern Congo, Chenard said.
Rwandan rebels, including members of the former Rwandan army and Hutu Interahamwe militia, have been based in eastern Congo since leading the 1994 genocide in Rwanda.
The Hutu forces fled into Congo after Tutsi-led rebels ousted the Hutu government that orchestrated the massacre of more than 500,000 people.
"The assailants hid in the cover of the forest along the road and opened fire on the two trucks (marked) with the UN insignia," said Chenard.
'Trails of blood'
There were 30 South African peacekeepers in the UN trucks and they returned fire, but did not pursue their attackers, Chenard said.
Other peacekeepers who arrived on the scene shortly after the attack said "there were trails of blood going deep into the forest," Chenard added.
There are some 10,800 UN troops
deployed in Congo
There are some 10,800 UN troops deployed in Congo to help implement peace deals that ended the five-year civil war in Africa's third-largest country.
The war in Congo erupted in August 1998, but ended when the main rebel groups and the government set up a transitional government last June.
The bulk of the peacekeeping force is deployed in eastern and northeastern Congo which have remained volatile, mainly because of attacks by rival tribal factions and the Rwandan insurgents.
Sunday's killing brings to at least 40 the number of UN staff who have died in Congo, since the UN peacekeeping mission began in late 1999.
Most fatalities were from vehicle accidents and illness, said Hamadoun Toure, a UN spokesman in the capital Kinshasa.