Mutilated bodies found in river near Burundi capital
Fourteen bodies, some of them mutilated with machetes, have been discovered in a river west of Burundi's capital.
At least one of the victims - whose identities are not yet known - was decapitated.
The brutality of the killings is feeding fears that a new rebellion is brewing in the country, which is still recovering from a 12-year civil war.
Seven people were killed in an attack last week, which the government has blamed on bandits, not rebels.
Elections earlier this year were undermined after opposition candidates refused to stand against President Pierre Nkurunziza, alleging fraud.
The main opposition leader, former rebel leader Agathon Rwasa, also disappeared before the June polls, and is thought to have fled across the border into the Democratic Republic of Congo.
The BBC's Prime Ndikumagenge in the capital, Bujumbura, says the bodies have been washing up on the banks of the River Ruzizi, close to the border with DR Congo, for the past week.
He says it appears the bodies were tied up before being thrown into the river.
There have been recent reports of heavily armed men with new military uniforms being seen in the north and west of the country.
The authorities said the corpses may be linked to last week's attack in the northern Rukoko area, which they believed was caused by bandits targeting farm workers and cattle.
"The government is resolved to track them down so that justice can apply to them a punishment equal to their crimes," a government spokesman said.
Some 300,000 people died in Burundi's ethnic-based civil war.
During the conflict, both President Nkurunziza and Mr Rwasa led mainly Hutu rebel groups fighting against the army, which was then dominated by the Tutsi minority.
Mr Rwasa initially refused to end the fighting when other warring factions set up a power-sharing government followed by elections in 2005.