UN Human Rights Commissioner Zeid Raad al-Hussein said on Friday they had documented 13 cases in Burundi of sexual violence in which security forces allegedly entered the houses of victims, separated the women and then raped or gang raped them.

He warned of "new and extremely disturbing patterns of violations" which had evidently been triggered by the attacks against three military camps on December 11.

Armed opposition fighters stormed the camps in Bujumbura and Mujejuru in order to seize weapons and free prisoners, an army spokesman said at the time.

As well as retaking the camps, government forces also responded by raiding areas known be to centers of the opposition. Residents reported summary killings and the discovery of dozens of bodies.

During the searches in the Musaga, Nyakabiga, Ngagara, Citboke and Mutakura neighborhoods of Bujumbura on December 11 - 12, police, army and Imbonerakure militia forces also arrested many young men who were later tortured, killed or taken to unknown destinations, Zeid said.

The UN says it is analyzing satellite images to investigate witness reports of at least nine mass graves in and around the capital Bujumbura, including one in a military camp, containing more than 100 bodies in total, all of them reportedly killed on the day of the attacks.

Cecile Pouilly, media officer for the UN Human Rights Council in Geneva, told DW they were calling for a credible, intensive and imediate investigation into the mass graves.

"The sites of the mass graves also need to be guarded, otherwise evidence which could shed light on the truth of what happened could go missing," she said.

Pouilly said the Council was shocked by the impunity prevailing in Burundi.

Ethnic dimension

One of the sexually abused women testified that her abuser told her she was paying the price for being a Tutsi. Another witness said Tutsis were being systematically killed, while Hutus were being spared.

Tension between majority Hutus and minority Tutsis led to a civil war in Burundi in which 300,000 people died. It ended in 2005.

The UN human rights chief warned of renewed violence between the two ethnic groups.

"All the alarm signals, including the increasing ethnic dimension of the crisis, are flashing red," Zeid said.

The UN rights office's Africa expert, Scott Campbell, said a team of three experts was ready to deploy to Burundi to investigate grave crimes allegedly committed during the crisis, but so far Bujumbura has not cleared the group.

Campbell said the UN rights office had "documented more than 3,000 arrests and that many had been released. But an unknown number had disppeared."

He said the investigators were planning to deploy on January 25th but were "still waiting for a government response."