President Pierre Nkurunziza has given armed gangs in Burundi 15 days to surrender to authorities and hand in their weapons or face unspecified action.

Nkurunziza was speaking during a visit to Mugamba district, some 65km southeast of the capital Bujumbura, where many local officials have been killed in recent weeks.

Tit-for-tat attacks between Nkurunziza's security forces and his opponents have escalated since April 2015 when he announced a disputed bid for a third term. He won re-election in July.

"We give them 15 days to surrender," Nkurunziza said of the armed gangs on Wednesday. "On the 15th (of June) we will put a stop to (the violence). I came to tell you that we won’t rest until security is restored."

The government conducted a similar arms mop-up in the capital in November.

READ: Burundi: Forced disarming going on, 97 released

Burundi police said in early May that more than 450 people had been killed in unrest that began a year ago. The United Nations says over 250,000 have fled the country since last April.

A new round of peace talks between the government and its opponents aimed at ending the political crisis was launched in the Tanzanian city of Arusha in late May. Previous discussions in Burundi and Uganda over the past year collapsed or stalled.

READ: Burundi peace talks going nowhere if govt stays picky