Unknown gunmen killed four people on Saturday night in two villages southeast of Burundi's capital Bujumbura.
The killings were the latest in a series of violent incidents which have rattled the small central African nation since a political dispute erupted a year ago.
Tit-for-tat attacks between President Pierre Nkurunziza's security forces and his opponents have been escalating since he announced a disputed bid for a third term in April 2015.
More than 400 people have been killed in the ensuing violence, according to estimates by the U.N. and rights groups.
Nkurunziza's opponents said a third term violated Burundi's constitution and a peace pact that ended the country's 1993-2005 civil war.
A constitutional court ruling, however, said Nkurunziza could extend his rule.
Citing that ruling, the government organized an election which the president won in July.
Etienne Nijimbere, a local government official, told Reuters on Sunday the gunmen had attacked two villages in Mugamba district, 60 kilometers southeast of Bujumbura, and killed four people.
Another source told Reuters that "some of the attackers were wearing military uniforms with hoods on [their] heads".
Nijimbere said all four people killed were likely targeted because they were members of the ruling CNDD-FDD party.
Late on Saturday a grenade was also thrown at a fuel reserve facility in Burundi's second largest town, Gitenga, although the blast caused no damage, according to a regional government official, Anicet Manirambona.
Three armed groups, including one led by officers that attempted a coup in May 2015, have launched armed rebellions against the government, officials say.
None have claimed responsibility for Saturday's attacks.
Burundi has accused neighboring Rwanda of supporting the anti-Nkurunziza rebels and there are concerns the violence could convulse the entire Great Lakes region, still haunted by Rwanda's 1994 genocide.
Rwanda and Burundi both have ethnic Hutu majorities and Tutsi minorities.
