Thousands of people have fled their homes in recent weeks
Burundi's army says it has killed 50 rebel fighters in the latest clashes near the capital, Bujumbura.
Spokesman Colonel Adolphe Manirakiza said the army responded after its patrol was ambushed. He said two soldiers were also killed.
This is the heaviest fighting since clashes resumed last month.
Meanwhile, rebel FNL spokesman says they will go to the capital next week to discuss reviving a ceasefire agreement signed more than a year ago.
"The Palipehutu-FNL has not lost the war but it wants peace," Pasteur Habimana told the AFP news agency.
He could not confirm the army's casualty figures.
"Those who died and were captured are Burundians. The army shouldn't be rejoicing so much at a time when everybody, including the international community, is seeking to rekindle peace efforts in our country," he said.
About 100 people have died since the FNL fired shells at Bujumbura three weeks ago.
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 BBC's Prime Ndikumagenge in Bujumbura says a ceasefire was agreed with the FNL in September 2006 but it has not been implemented.
The rebels did not confirm the army's casualty figures
FNL leaders in exile in Tanzania want full immunity from prosecution and a share of government jobs.
But this would require constitutional changes which the government - wracked by a parliamentary crisis - does not have the power to deliver.
Earlier this week, Tanzania gave the rebel leaders a 10-day deadline to return to Burundi and resume negotiations.
Correspondents say the latest fighting will only deepen the distrust that has so far prevented a final peace deal.
Tanzania and Uganda, which helped broker the ceasefire agreement, are becoming increasingly impatient with the rebels, seeing them as the final obstacle to lasting peace and stability in Burundi.
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.