A rights group said Friday it had seen evidence indicating possible mass graves in Burundi as unrest escalated with the arrest of 17 people in a security sweep, including two foreign journalists.

Satellite images, video footage and witness accounts show that dozens of people allegedly killed by Burundian security forces in December were later buried in mass graves, Amnesty International reported Friday.

The rights group reported that there are five possible mass graves in the Buringa area, on the outskirts of the capital, Bujumbura, which has been wracked by violence as the security forces go into neighborhoods seen as opposition strongholds. Two journalists on assignment for the French newspaper Le Monde were among 17 people swept up in a military operation Thursday, said Moise Nkurunziza, a deputy spokesman for Burundian police.

Le Monde demanded the immediate release of Jean-Philippe Remy and British photojournalist Philip Edward Moore.

Witnesses described how police and local officials scoured Nyakabiga and other neighborhoods in Bujumbura to retrieve the bodies of those who were killed and took them to undisclosed locations, according to Amnesty International.

"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," the group said.

Earlier this month, U.N. human rights chief Zeid Raad al-Hussein also called for an urgent investigation into the alleged existence of mass graves following the violence in December. Zeid said at the time that the "increasing number of enforced disappearances, coupled with allegations of secret detention facilities and mass graves is extremely alarming."

Burundi's government dismissed those allegations, saying they were based on false information supplied by the regime's opponents.

In coordinated attacks, gunmen stormed three military installations in Burundi on Dec. 11. The next day, 28 people were found shot dead in three Bujumbura neighborhoods. An eyewitness told the AP some of the dead had their hands tied behind their backs. Another witness blamed government security forces, saying they went after the victims in door-to-door searches.

President Pierre Nkurunziza's decision to seek re-election to a third term last April touched off street protests that led to a failed coup in May and a rebellion that has left the country on the brink of civil war.

The Burundian government has rejected the proposed deployment of African Union peacekeepers in Burundi, saying they will be treated as an invading force.

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Muhumuza reported from Kampala, Uganda.