The UN rights chief called Thursday for the Ethiopian authorities to conduct "prompt, impartial and thorough" investigations into weekend attacks in the west of the country that have killed hundreds of people.
"I am horrified by the senseless killing as well as the forced displacement of the local population in the attack on Tole village," Michelle Bachelet said in a statement.
She said her staff had spoken with witnesses to the June 18 attack in the Oromia region who said armed individuals descended on Tole and began shooting randomly, leaving hundreds dead.
Most of the victims were women and children, the statement said, adding that at least 2,000 other people had been forced to flee their homes.
"The armed individuals also reportedly burned down several houses during the four-hour assault," it said.
Ethiopia's parliament also instructed Thursday the Ethiopian Human Rights Commission, a state-affiliated independent body, to investigate the killings which it said were carried out by the Oromo Liberation Army.
"A terrible and inhumane atrocity has been committed by the (OLA) on innocent citizens" in parts of Oromia and the neighbouring Gambella region, it said.
Survivors of the Tole attack told AFP that hundreds of people belonging to the Amhara ethnic group were killed by rebels, with bodies still lying on the streets after hours of violence.
The Oromia regional authorities have also blamed the attack on the OLA, which is branded a terrorist organisation by the Ethiopian government.
But a spokesman for the rebel group blamed the region's own pro-government militia for the killings and called for an independent investigation.
"I call on the Ethiopian authorities to ensure that investigations are promptly launched into the attack and to ensure that victims and their families have a right to truth, justice and reparations, including by holding those responsible to account," Bachelet said.
She also said an unspecified number of people were reported to have been abducted during the attack and that their whereabouts remained unknown.
"I call on the authorities to take all necessary and lawful measures to ensure those abducted regain their freedom," Bachelet said.
Abune Mathias, the patriarch of the Ethiopian Orthodox Church, also voiced his sorrow at the "terrible murders" in a statement on Thursday.