U.N. officials have started an investigation after militiamen in Congo's northeastern district of Ituri killed a U.N. military observer on Thursday. The gunmen ambushed a U.N. convoy near the town of Katoto, about 25 kilometers north of the district capital Bunia.

The ambush took place on Thursday afternoon, as U.N. troops left the town of Katoto where they had been sent to investigate a recent round of violence in the bush, suspected to be fighting between rival armed ethnic factions. Two U.N. vehicles were hit by fire from the bush, as the convoy left Katoto, killing a military observer from Kenya. U.N. troops returned fire, and an attack helicopter was called in to disperse the assailants.

The killing of the U.N. military observer comes after a spate of attacks against U.N. convoys in the troubled district of Ituri in the last few weeks. U.N. troops took over from a French led force in Bunia in September, with the mandate to deploy into Ituri, a district roughly the size of Liberia, but have struggled to deploy to the furthest reaches of the district, where massacres still take place, and warlords roam beyond U.N. control.

Since 1999, mineral-rich Ituri has been wracked by conflict between ethnic Hema and Lendu militias, supported by Rwanda, Uganda and members of the former government of Congo.

The conflict in Ituri has claimed over 50,000 lives so far, and is a stain on Congo's new government of national reconciliation.

U.N. officials say the latest attack may well be the result of collaboration between a faction of the principal Hema militia groups and Lendu fighters.

While the U.N. says it is still going ahead with its plans to deploy throughout the entire Ituri district, the attacks against its convoys are gradually creeping back toward the capital Bunia.