Survivors of the rebels' massacre in western Ethiopia counted 54 bodies in a schoolyard on Sunday, the latest attack in which ethnic minority members were deliberately targeted, Amnesty International said.
Human rights groups are asking why, just hours before attackers moved in and targeted ethnic Amharas, federal soldiers left the region.
The Prime Minister of Ethiopia, Abiy Ahmed, denounced the identity-based killing of people, adding that security forces were deployed to the area and "started taking action."
Ethnic violence in Ethiopia poses the greatest challenge yet to the prime minister, who for his sweeping political reforms was the Nobel Peace Prize winner last year.
Abiy said in a Facebook post, "Ethiopia's enemies are vowing to either rule the country or ruin it, and they are doing everything they can to accomplish this." "One of their tactics is to arm civilians and carry out identity-based barbaric attacks. This is heartbreaking (for me)."
A rebel group, the Oromo Liberation Army, was blamed by the Ethiopian government for the attacks in the far western part of Oromia, in an area bordering South Sudan and a few hundred kilometers from Addis Ababa, the capital.
Arara Merdasa, the head of the Oromia region police commission, told the state broadcaster that the death toll was 32, and "about 200 families have fled the area."
Amnesty International was told by survivors of the attack in the Guliso District of the West Wellega Zone that federal troops had abruptly withdrawn and the rebels arrived hours later, identifying themselves as the OLA and announcing that they now controlled the area.
"Militants gathered and killed people who were unable to flee, mostly women, children and the elderly," the Amnesty statement said. Survivors were hiding nearby in a forest. One told the human rights group that he had found the bodies of his brother, his sister-in-law, and three children with bullet wounds in the schoolyard.
"The fact that this horrendous incident took place shortly after the abrupt withdrawal of government troops from the area under unexplained circumstances raises questions that need to be answered," said Deprose Muchena, regional director of Amnesty International.