Witnesses say two people have been killed in overnight attacks that they believe were carried out by Burundian police.

Witnesses in Jabe neighbourhood in the nation's capital Bujumbura, reported intense gunfire on Saturday night in an apparent police crackdown on areas seen as hosting anti-government protesters.

Mohamed Masumbuko said his brother, Masumbuko Prime Abdul, was killed in a grenade attack, his "face completely shredded".

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Another victim, Patrick Ndikumana, was first hit by a bullet but was killed in a subsequent grenade attack, according to witness Joshua Ngendakumana, the AP news agency reported.

It was not immediately possible to get a comment from the police, who have previously blamed an escalation of violence on protesters opposed to a third term for President Pierre Nkurunziza.

Parliamentary elections are scheduled to take place Monday despite the violence.

On Saturday, attackers in Burundi set fire to a building storing election materials following weeks of violence over Nkurunziza's bid to stay in power.

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

Burundi plunged into a political crisis after Nkurunziza declared he would stand for a third-term as leader despite opposition claims that the bid violated a constitutional limit to serve no more than two terms as president.

In May army officers opposed to Nkurunziza launched a coup attempt that failed to dislodge him, but it was quickly put down by loyalists.

Source: Agencies