GENEVA — Burundi’s security forces are engaging in gang rape during a crackdown on political opponents that has included a sharp rise in torture, killings and disappearances in the past month, the top United Nations human rights official said on Friday, sounding an alarm over the increasingly ethnic character of the violence.

The charge of sexual violence adds a brutal new twist to a deepening nine-month crisis that has put Burundi, a poor, tiny nation in Central Africa, on the agenda of the United Nations Security Council and the African Union.

The United Nations said that at least 439 people, including many critics and people suspected of being opponents of the government, had been killed, and that close to a quarter of a million had fled to neighboring countries since President Pierre Nkurunziza decided in April to run for a third term.

“All the alarm signals, including the increasing ethnic dimension of the crisis, are flashing red,” Zeid Ra’ad al-Hussein, the United Nations high commissioner for human rights, said in a statement released in Geneva. He reported that his office had identified mass graves, including one in a military camp.