"Armed bands [rebels] attacked in the three communes overnight on Monday. They killed 13 people, but the army didn't kill anybody," said army spokesman Lieutenant-Colonel Isaie Nibizi.
The aid workers, who declined to be identified, said troops killed more than 100 people at Mugara and 51 people including women and children on the hills near Maramvya in Burambi commune Monday.
A Defense Ministry spokesman said Tuesday that 50 gunmen killed the 13 and destroyed a health center in Mugara, six kilometers east of Lake Tanganyika bordering Zaire, when most of its male residents were out on a night security patrol.
The Tutsi-dominated army, which seized power in July last year, is fighting a vicious war against Hutu rebels. Both sides are frequently accused by human rights groups of massacring civilians.
"The rebels attacked at three places. They attacked a camp of displaced people at Maramvya at Burambi commune and killed three displaced people, and the army reacted by killing 51 people including women and children," said one of the aid workers.
"At Mugara in Rumonge commune the rebels killed 12 people including a protestant pastor, and the army reacted by killing more than 100 people on the hills," the aid worker added.
Aid workers said more civilians were killed at Mubuga in fighting between troops and rebels but had no exact figures.
Dozens of wounded civilians were treated Friday at a hospital in central Bujumbura, about 180 kilometers north of the sites of the killings.
More than 150,000 people -- mainly civilians -- have been killed in Burundi since October 1993 when soldiers murdered the country's first elected Hutu president in an attempted coup.
Conflict in the southwestern province of Bururi has increased this year as rebels move south after being driven out last November from bases in refugee camps in Zaire and from their stronghold in northwestern Burundi's dense Kibira forest.
About 4,000 people -- both Hutus and Tutsis -- have left their homes this month in Bururi, an eyewitness reported.
"They prefer to ask for food handouts from the different churches rather than return to their homes because they are afraid of being caught up in armed attacks," the witness said.
The UN human rights agency said in January about 1,000 people were killed since last December, most by the army.
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