The dead included 22 civilians, 19 rebels and two soldiers. There were at least 34 wounded.
Around 200,000 people are thought to have died in six years of civil war between Hutu rebels and the Tutsi-dominated army in Burundi, with over a million people forced from their homes.
It is a bitter ethnic conflict which mirrors the divisions in its northern neighbour Rwanda, where extremist Hutus butchered 800,000 Tutsis and moderate Hutus in 1994.
The latest attack came on the military camp at Kamenge on the outskirts of Bujumbura on Tuesday evening, with two soldiers and 10 rebels killed, according to the army.
The rebels also attacked the Rutana camp for internally displaced refugees in the southeast of the tiny Central African country on Monday evening, killing five civilians and losing nine of their own number, officials said.
In another attack on the Matongo displaced camp in northern Kayanza province on Saturday, the rebels killed 17 civilians, local government officials told Reuters.
On Wednesday, former South African President Nelson Mandela was appointed as the new mediator in Burundi's fragile peace process, to replace Tanzania's former President Julius Nyerere who died in October.
In Johannesburg, Mandela welcomed the appointment.
"Mr Mandela is overwhelmingly pleased to take on the appointment," his spokeswoman Zelda La Grange told Reuters, adding that the former president would begin working towards peace this weekend with various meetings already scheduled.
But the latest attacks underscore the scale of the task facing the 81-year-old Nobel Peace Prize winner, with the main rebel group, the Forces for the Defence of Democracy (FDD) not even participating in the peace talks.
In response to an escalation of the war in the last six months, the army has forced hundreds of thousands of Hutu peasants into regroupment camps, ostensibly for their own protection.
It is a policy which has been condemned by the international community and by aid workers who warn of a possible humanitarian crisis in the camps where sanitation is poor and food increasingly scarce.
- Reuters - Thomson Reuters Foundation
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