Human Rights Watch says members of the youth league of Burundi's ruling party have tortured and brutally killed scores of people across the country in recent months.

In a report released Thursday, the rights group said members of Imbonerakure carry out violent crimes with impunity because President Pierre Nkurunziza's government is unwilling to prosecute or rein in the youth group.

Imbonerakure members have been frequently mentioned among the perpetrators of violence in Burundi since April 2015, when Nkurunziza announced he would seek a disputed third term.

Hundreds of people have since died, with the security forces blamed for most of the killings.

A spokeswoman for the ruling party denied the allegations, telling Human Rights Watch that Imbonerakure carry out political activities "calmly and serenely" and do not make arrests.