Bujumbura - Gunmen from an armed splinter group of Burundi's main opposition party claimed they had killed 12 soldiers Thursday, although the army denied the reports, amid growing tensions in the central African nation.
A dissident wing of the National Liberation Forces (FNL) said its fighters on Wednesday attacked two bases at Burniga, some 14km from the capital Bujumbura.
"Four soldiers were killed at the first base and eight others at the second," rebel spokesperson Eraste Dondogori told AFP. "We did not suffer any losses because they were taken completely by surprise."
But army spokespeson Gaspard Baratuza denied the claim, insisting "the attackers fired for a short time... but there was no damage."
Witnesses however spoke of a heavy attack that lasted nearly an hour.
"The rebels were many in number, attacking with rocket launchers, grenades and automatic weapons," one witness said, speaking on condition of anonymity.
The same group has claimed similar attacks in recent weeks, amid growing tensions ahead of elections scheduled for 2015, in which President Pierre Nkurunziza is expected to campaign for a third term in office.
Since a controversial series of polls won by the ruling party in 2010 after an opposition boycott, sporadic attacks have stoked fears that Burundi's 1993-2006 civil war could resume.
The FNL, whose full name is Party for the Liberation of the Hutu People, was the last ethnic Hutu rebel group to sign a peace accord with the government in 2006.
The government has been reluctant to label the unrest a "rebellion", instead insisting the attacks are the work of "bandits".
Burundi's history is marred by bitter ethnic killings and civil war.
This week the leader of a key opposition party, Alexis Sinduhije of the Movement for Solidarity and Development (MSD), was charged with rebellion along with 71 supporters after violent clashes last week.
Sinduhije, who is on the run, faces life in jail if found guilty.
Last month leaders of the opposition Uprona party were arrested, threatening to upset a delicate power-sharing arrangement between Burundi's majority Hutu and minority Tutsi communities, who are still struggling to reconcile after decades of conflict.