Attackers in Burundi have set fire to a building storing election materials, two days ahead of key polls and following weeks of violence over President Pierre Nkurunziza's bid to stay in power.

National electoral commission officials confirmed the attack in the northeast Ntega district, some 200km from the capital Bujumbura, but there was no indication who was responsible.

"A group of unidentified young people took advantage of the police who were sleeping on duty and torched a building housing election material," local governor Reverien Nzigamasabo told the AFP news agency. "Part of the ballot boxes and voting booths were burned, but people were able to save the rest."

Soldiers fired shots at the gang to chase them away, police said.

Opposition boycott

Two grenades were also hurled overnight Friday in Bujumbura, witnesses said, but unlike a string of similar attacks, no one was hurt. 

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 on Friday said it was boycotting polls, claiming it is not possible to hold a fair vote.

Burundi's journalists refuse to stay silent

UN Secretary-General Ban Ki-moon on Friday 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.

Opponents say Nkurunziza's bid for another term is unconstitutional and violates a peace accord that ended 13 years of civil war in 2006.

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, on Friday after a letter signed by all the country's opposition groups was handed to the election commission.

Civil society groups backed the move in a joint statement calling on voters to skip the "sham elections" and urging the international community "not to recognise the validity" of the polls.

"Thousands of Burundians have fled the country, a thousand peaceful demonstrators were arrested, tortured, and are currently languishing in jail," the statement said.

Source: Agencies