Three ruling party activists have been shot dead by gunmen in a bar in eastern Burundi, police say.
Five men dressed in army fatigues tied up the CNDD-FDD party supporters before killing them on Sunday night.
They then went to the CNDD-FDD office in Gisuru, near the border with Tanzania, and burned the party flag.
Party spokesman Onesime Nduwimana blamed "the extremist fringe of the opposition" bent on sabotaging elections due later this year.
He told the AFP news agency that he feared they wanted to destabilise the country by "sowing hatred" and provoking revenge attacks.
The attack follows heavy clashes last week in north-western Burundi between the army and rebels who officials say crossed from neighbouring Democratic Republic of Congo.
On Monday, an army spokesman said 95 fighters from the so-far unidentified rebel group had died during the fighting in Cibitoke province.
Col Gaspard Baratuza said the motive of the rebels was not immediately clear but there were suspicions the group might have been planning to disrupt the elections.
Burundi is recovering from a brutal civil war that ended with President Pierre Nkurunziza's election in 2005.
Mr Nkurunziza, a former rebel leader and CNDD-FDD leader, is widely expected to seek another term in office but has not confirmed whether he will do so.
The ruling party and its opponents have disagreed over whether President Nkurunziza can run for a third term.
Analysts say the peace accord that ended the conflict states there is a two-term limit, but the constitution is open to interpretation.
Last year, Amnesty International said the ruling party was waging a "relentless campaign of intimidation" against its critics ahead the polls.