At least 700 people have been killed in clashes between President Pierre Nkurunziza's supporters and his opponents since April 2015.

BUJUMBURA - A grenade attack killed eight people and wounded 50 others in a village in Burundi's northern Kayanza province late on Sunday, police said.

Four of the victims died at the scene and the other four succumbed to their injuries, police spokesman Pierre Nkurikiye said.

The assault occurred around 6:30 pm local time in the village of Shinya on Sunday, Nkurikiye told reporters in the capital, Bujumbura, on Monday. He called the assault "a terrorist attack".

"An unknown person threw a grenade among the population," he said.

Nkurikiye did not say who might have carried out the assault, but police usually use the phrase "terrorist attack" to indicate armed opponents of President Pierre Nkurunziza.

At least 700 people have been killed in clashes between Nkurunziza's supporters and his opponents since April 2015, when the president said he would run for a third term in office. He was re-elected in July 2015 in a vote largely boycotted by the opposition.

In a tweet on Monday, Nkurunziza offered condolences "to the families who lost their next of kin in the terror attack ... The perpetrators will surely be punished."

Nkurunziza's opponents say his bid for a third term violated the country's constitution and the terms of a peace deal that ended a previous rebellion. The president cited a constitutional court ruling that his bid was legal.

Burundi's growing insecurity has spurred some alarm in the region because it has the same ethnic mix of majority Hutus and minority Tutsis as neighbouring Rwanda, where 800,000 people were killed in ethnic violence in 1994.

An estimated 400,000 Burundians have so far been displaced by the fighting between Nkurunziza's backers and his opponents.