National electoral commission officials confirmed the attack in the northeast Ntega district, some 200 kilometres from the capital Bujumbura, but there was no indication who was responsible.
Parliamentary and local elections are set to be held in the central African nation on Monday, and a presidential vote on July 15. The opposition yesterday said it was boycotting polls, claiming it is not possible to hold a fair vote.
Soldiers fired shots at the gang to chase them away, police said.
Two grenades were also hurled overnight yesterday in Bujumbura, witnesses said, but unlike a string of similar attacks, no one was hurt. Gunfire was also heard in the capital overnight, as it has been for recent days.
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UN Secretary General Ban Ki-moon yesterday called for them to be delayed after the opposition said they would not take part in the polls, which come as Burundi faces its worst crisis since its civil war ended nine years ago.
Around 70 people have been killed in weeks of demonstrations that have been brutally suppressed, triggering an exodus of over 127,000 people into neighbouring countries.
Burundi was plunged into turmoil in late April when Nkurunziza launched his drive for a third consecutive five-year term, triggering widespread protests and a failed coup.
"All the opposition have unanimously decided to boycott the elections," said Charles Nditije, a key opposition leader, yesterday after a letter signed by all the country's opposition groups was handed to the election commission.
"Thousands of Burundians have fled the country, a thousand peaceful demonstrators were arrested, tortured, and are currently languishing in jail," the statement said.