The United Nations says it has evidence that Burundi's security forces gang-raped women while searching the homes of suspected opposition leaders.

Security forces separated the women and raped them, the UN said in a statement. It said it had documented 13 cases of sexual violence.

Forces also kidnapped, tortured and killed dozens of young men, it said.

Violence has escalated in Burundi since President Pierre Nkurunziza announced he would run for a third term.

He survived a coup attempt in May, and secured a third term in disputed elections in July.

The abuses took place immediately after rebel attacks in December against three military camps in the country's capital, Bujumbura, the UN's human rights chief, Zeid Ra'ad al Hussein, said in a statement.

The UN believes the army is taking revenge for the attacks.

Mr Hussein also called for an investigation into reports that nine mass graves were found in and around Bujumbura, warning about the "increasing ethnic dimension of the crisis".

Some witnesses say that violence from security forces was directed at the Tutsi minority.

Burundi has been plagued by tension between Tutsis and Hutus since independence in 1962.