Amnesty International says new satellite images, video footage and witness accounts show that dozens of people killed by Burundian security forces in December were later buried in mass graves.

The rights group reported Friday there were five possible mass graves in the Buringa area, on the outskirts of the capital Bujumbura.

The group said: "The imagery, dating from late December and early January, shows disturbed earth consistent with witness accounts. Witnesses told Amnesty International that the graves were dug on the afternoon of Dec. 11, in the immediate aftermath of the bloodiest day of Burundi's escalating crisis."

Earlier this month, the U.N. rights chief also called for an urgent investigation into the alleged existence of mass graves.

Burundi's government dismissed the allegations, saying they were based on false information.