Fighting has broken out between Burundi's last active rebel group and government forces, a day after rebels announced a unilateral ceasefire.
The main FDD rebels are being integrated into the army, leaving the FNL isolated
Both sides accuse the other of starting the clash.
The National Liberation Forces (FNL) announced a truce on Thursday, but warned it would fight back if attacked by government troops.
The government welcomed the rebel call, though the FNL has still to agree to talks aimed at ending the 10-year war.
About 300,000 people have been killed during the conflict between ethnic Hutu rebels and an army dominated by the Tutsi minority.
FNL spokesman Pasteur Habimana said that army forces attacked them in Burima, south east of the capital Bujumbura.
"They are firing shells. The fighting is going on," Mr Habimana told AFP news agency.
But army spokesman Adolphe Manirakiaza,
accused the rebels of attacking their supply patrols.
"It was the FNL that violated the ceasefire they declared. Clearly, we have to defend ourselves," said Mr Manirakiaza.
The BBC's Robert Walker in the Great Lakes region says despite the FNL's declaration of an end to hostilities, deep mistrust remains between the government and the rebels, and analysts say there is still a long way to go before any lasting peace deal can be struck.
In the past, the FNL said it would only negotiate with the Tutsi leadership of the army who, it claims, hold the real power in Burundi.
But the FNL has found itself increasingly isolated since the power-sharing agreement late last year between the government and the larger Hutu rebel group, the Forces for Defence of Democracy (FDD).