At least eight Burundian soldiers were killed and dozens of others missing following an ambush on Sunday about 100 kilometres north of the capital Bujumbura, according to military sources.
It is believed that there were about 90 soldiers at the base, which is near the border with Rwanda, before the raid happened.
The Burundian Defence ministry on Monday said that a group armed with rifles from Rwanda attacked a position of Burundian soldiers on Mount Twinyoni.
The ministry spokesman, Major Emmanuel Gahongano, who spoke on state TV, also said that the armed group withdrew to Rwanda.
The spokesman did not elaborate on the casualties or the identity of the attackers.
15 soldiers were later found alive with some nursing injuries, AFP reports.
Military officers who spoke to AFP on condition of anonymity termed this as one of the largest and deadliest attacks for several years in Burundi.
According to the military sources, the attackers were well armed with equipment such as bullet-proof vests and night-vision goggles.
Burundi has been plagued by political violence following President Pierre Nkurunziza’s announcement in April 2015 that he would seek a disputed third term. Nkurunziza won re-election despite widespread protests.
