Burundian soldiers were attacked in a night jungle ambush near the border with Rwanda, Burundi's defence ministry said, with military sources on Tuesday reporting at least eight soldiers' deaths.
Dozens more soldiers were missing in the ambush on their base, one of largest and deadliest attacks for several years, senior army officers said Tuesday, speaking on condition of anonymity.
"A group armed with rifles from Rwanda attacked a position of Burundian soldiers on Mount Twinyoni," defence ministry spokesman Major Emmanuel Gahongano said on state television on Monday. "This armed group has withdrawn to Rwanda."
He did not give details of casualties or the identity of the attackers.
Burundi has been in crisis since President Pierre Nkurunziza defied constitutional limits to seek a third term in office, winning re-election in 2015.
Burundi has repeatedly accused neighbouring Rwanda of supporting rebel groups in its territory, a claim Kigali denies.
Rwanda on Tuesday denied any role in the attack.
"It is not true that the attacks were made from people who came from Rwanda," Olivier Nduhungirehe, State Minister for Regional Affairs, told AFP.
"These are unfounded allegations being made from Burundi -- as they have done previously for the last four years. We have other things to do."
The attack, some 100 kilometres (60 miles) north of the capital Bujumbura, in thick forests 10 kilometres from the Rwandan border, took place in the early hours of Sunday morning.
Around 90 soldiers were reported to be in the base before the attack.
But when reinforcements arrived hours later, they found only the bodies of eight comrades, including of the company commander, a senior officer told AFP.
Later, 15 soldiers were found alive, some of them wounded.
"The rest of the company is still missing," the officer said. Their fate is unknown.
The military source reported that attackers were well-equipped.
"Our soldiers were surprised by assailants wearing bullet-proof vests and night-vision goggles, which completely wiped out the position," the officer said, a report confirmed by two other military sources.
"We believe that it is not mere rebels who are responsible for it."
No Burundian armed group has claimed responsibility.