Burundi's army says the last active rebel group has fired mortar bombs into the capital as peace talks that began on Monday continue in Tanzania.
Fighting between the government and FNL resumed in May 2005
The army said at least one civilian had been wounded in the attack on Bujumbura that happened late on Tuesday night.
The Hutu National Liberation Forces (FNL) are still outside a power-sharing agreement to end a 13-year civil war.
FNL leader Agathon Rwasa said he had no knowledge of the attack but his troops had the right to defend themselves.
Burundi has long been dominated by its Tutsi minority but now has a Hutu leader - ex-rebel Pierre Nkurunziza, elected last year.
Fighting between the government and FNL resumed in May 2005, only one week after the two sides signed a ceasefire.
South African Security Minister Charles Ngqakula is mediating at the talks in Dar es Salaam.
The BBC's Prime Ndikumagenge in Burundi says the FNL have imposed different conditions from the other rebel groups, maintaining that the problem in Burundi is between Hutus and Tutsis and demanding direct negotiations with Tutsi army or civilian leaders.
The current round of talks had been delayed by a disagreement over who would broker the talks, but both sides have now agreed to Mr Nqakula's mediation.
Some 300,000 people were killed in the civil war, which was sparked in 1993 by the assassination of Burundi's first Hutu head of state and democratically elected president, Melchior Ndadaye.