The UN human rights chief is warning of a sharp increase in the use of torture in Burundi, saying at least 345 new cases have been reported since the beginning of the year.

Zeid Ra'ad Al Hussein said Monday the upsurge was a clear indicator of the widespread and growing use of torture by government security forces. He also expressed concern over secret government detention facilities.

Zeid also deplored the increase in attacks by unidentified armed men, reportedly linked to rebel groups, and condemned the targeting of members of the ruling party.

Burundi has been hit by unrest since April 2015, when President Pierre Nkurunziza announced his decision to seek a third term, which he eventually won.

UN spokesman Stephane Dujarric quoted his statement at UN headquarters in New York.