United Nations troops killed up to 38 militia fighters during a raid by hundreds of peacekeepers backed by helicopter gunships in the Ituri district of eastern Congo yesterday.
"It was serious fighting and 38 militiamen were killed," a UN source said.
The clash came a day after a UN deadline expired for voluntary disarmament by militias, who have killed hundreds of civilians in the mineral-rich region.
A UN spokesman said: "Our troops do not fire unless fired upon so the militia obviously shot first."
Earlier, the UN mission said South African, Bangladeshi and Pakistani peacekeepers exchanged fire with gunmen as they searched two militia camps 25 miles southwest of Bunia, capital of Ituri.
The United Nations now has 16,700 soldiers in Congo, making it the largest peacekeeping mission in the world.
It has been criticized in the past for failing to rein in the brutal militias, who have carried out systematic attacks on civilians, kidnapping, decapitating and torturing their victims.
But after nine Bangladeshi peacekeepers were killed in February - the worst single loss suffered by the peacekeeping mission since it began in 1999 - the UN force stepped up military operations against armed gangs.
Ethnic warfare has killed more than 60,000 people in Ituri since 1999. Fighting between Hema and Lendu militias has intensified since last December, hundreds have been killed and around 100,000 have fled their homes.