Burundians had hoped that the years of war were over
Burundi's army has launched a counter-attack in the hills around Bujumbura after the capital was shelled by suspected rebels.
Four soldiers and 10 rebels were killed in the clashes, the government says.
A BBC correspondent in the city says the sound of explosions and gunfire could be heard throughout the night.
Peace deals have been signed with most of Burundi's rebel groups - including one which now forms the government - except the FNL, which remains active.
The military's deputy spokesman Colonel Justace Ciza said the fighting had broken out in Bubanza, 50km (30 miles) north-west of the capital, Bujumbura.
"FNL rebels shelled mortar bombs and threw hand grenades on our different positions... but we retaliated," he said, Reuters news agency reports.
He also told the AFP news agency there were clashes in numerous locations around the city following a "major attack" by the rebels.
"The FNL has shown they are not commmitted to peace," government spokesman Hafsa Mossi told the BBC's Focus on Africa programme.
But an FNL spokesman said the army had started the fighting.
"This was self-defence," said FNL spokesman Pasteur Habimana from Dar Es Salaam in Tanzania.
The BBC's Prime Ndikumagenge in Bujumbura says a ceasefire was agreed with the FNL in September 2006 but it has not been implemented.
Witnesses have told him they saw troops and armoured vehicles heading to the hillsides north of the capital, shortly before the shelling started late on Thursday.
Ex-rebel Pierre Nkurunziza was elected president in 2005 under a deal to end years of conflict between the Tutsi army and Hutu rebels.
More than 300,000 people died in the war sparked in 1993 by the assassination of Burundi's first Hutu head of state and democratically elected president, Melchior Ndadaye.