Nine mass graves holding 100 bodies total have been found, including one on a Burundi army base, said U.N. High Commissioner for Human Rights Zeid Ra'ad Al Hussein. Thirteen cases of sexual violence against women have been reported, he said.
Witnesses said the army carried out retaliatory attacks after street violence in mid-December. Zeid said those clashes "appear to have triggered new and extremely disturbing patterns of violations."
Zeid said he's heard reports of kidnapping, secret detention centers and mass graves.
"My office is analyzing satellite images in an effort to shed more light on these extremely serious allegations," Zeid said.
Zeid said the sexual violence occurred in neighborhoods perceived as supportive of opposition forces.
"The pattern was similar in all cases: Security forces allegedly entered the victims' houses, separated the women from their families, and raped -- in some cases gang-raped -- them," Zeid said.
"New cases of sexual violence have continued to emerge since mid-December, including the reported rape of five women in a single house during a search operation in Bujumbura Mairie province."
Zeid said reports of torture also are increasing.
Burundi has experienced months of violence since President Pierre Nkurunziza announced in April that he would run for a third term
. That seemed to be a clear violation of the agreement that ended the country's civil war and Burundi's Constitution. He won re-electio
n in July.
The subsequent unrest has left scores dead and caused more than 170,000 people to flee
In the country's civil war, which lasted from 1993 until 2003, an estimated 300,000 people were killed. Burundi has about 10 million people.