Seven people have died as United Nations led troops battle with militia in north-eastern Democratic Republic of Congo with helicopter gunships.
Many militia have disarmed, but thousands remain in the bush
The Lendu militia group are refusing to be integrated into the Congolese army in line with a 2002 peace deal, a UN spokesman in the area said.
Meanwhile, the UN Security Council has authorised individual sanctions on leaders of foreign armed groups there.
The east has been out of the control of central government for many years.
Despite the end the conflict, bands of militia groups still terrorise civilians and use the rich minerals and timber of the region to finance their operations.
Neighbouring countries were drawn into DR Congo's brutal five-year war in which 3m people were killed.
Among the rebel groups targeted by the sanctions are those formed by ethnic Hutu extremists, who fled to DR Congo after the Rwandan genocide of 1994.
Last week, the UN says the Lendu militia attacked electoral officials attempting to prepare for Sunday's referendum on a new constitution.
"[The UN] sent reinforcements to the area and during the clashes that followed, one soldier and six militiamen were killed," UN spokesman Major Hans-Jakob Reichen told AFP news agency.
The confrontation dubbed Operation Eden, involving some 1,500 Congolese troops and more than 350 UN soldiers, is continuing north of Bunia, near the Ugandan border.
About 15,000 fighters in the region have already disarmed, but several thousand are believed to be still the bush, Reuters reports.
Some 15,000 UN troops are in DR Congo to help restore stability and organise elections next year.
This week, early results indicate that Congolese overwhelmingly backed a new post-war constitution in the referendum, which will pave the way to next year's polls.