At least 22 people have been killed in clashes between Burundi's army and members of an unidentified rebel group near the Democratic Republic of Congo border, sources have said.
"The security forces are still combing the area where there was fighting and the valleys where the members of this armed group are hiding," a top Burundian army official told AFP on Wednesday, the day after the rebel group launched the attack on the central African nation.
Burundian officials and witnesses said the group of around 200 rebels crossed into Cibitoke province north of the capital Bujumbura overnight on Monday.
The rebels crossed into the country from Democratic Republic of Congo's eastern Kivu region, an unstable and resource-rich area that is home to dozens of rebel groups.
Security forces then fought to prevent the group from reaching the Kibira forest, an area used in the past by rebel groups as a base to stage further attacks inside Burundi.
Previous attacks in Burundi's border region have been claimed by a splinter faction of the National Liberation Forces (FNL), whose full name is Party for the Liberation of the Hutu People.
The main body of FNL, a group notorious for singing hymns as they carry out attacks, signed a peace deal with the Burundian government in 2009 and has since become a political party.
The rebels who still fight on have claimed a string of attacks this year, most recently in October when they claimed to have killed six soldiers, and vowed to increase their raids ahead of presidential elections in June 2015.
The group, however, has denied they were behind the latest attack.
Burundi, a small nation in Africa's Great Lakes region, emerged in 2006 from a brutal 13-year civil war and its political climate remains fractious ahead of the polls.