Ethiopia's prime minister and a rebel group have traded blame for a mass killing in Oromiya, the country's most populous region, where hundreds of people have died in recent months in escalating violence between rival ethnic groups.

The latest killings took place on Monday in two villages in Kellem Wollega zone, around 400 km west of the capital Addis Ababa, the state-appointed Ethiopian Human Rights Commission (EHRC) said.

Both the EHRC and Prime Minister Abiy Ahmed blamed the Oromo Liberation Army (OLA), a banned splinter group of an opposition party, for the killings, which Abiy called a "massacre".

OLA spokesperson Odaa Tarbii rejected the accusations, saying government-allied militias were responsible for the slaughter, while federal troops recently deployed in the area did nothing to stop it.

Reuters was not immediately able to verify the claims of either side, with most communications to the affected area apparently down on Tuesday.

Oromiya's regional administration spokesman did not immediately respond to requests for comment and government spokesperson Legesse Tulu did not provide details on casualties.

Oromiya, which accounts for more than a third of Ethiopia's population of around 110-million, has experienced spasms of ethnic violence for many years, often rooted in grievances about political marginalisation and neglect by the central government.

Violence has escalated since the OLA formed an alliance last year with the Tigray People's Liberation Front (TPLF), which has been fighting government forces in the north of the country since November, 2020.

While Abiy and forces loyal to him say they want to end the fighting in Tigray, recent bloodshed in Oromiya is a reminder that ethnic rivalries across the country threaten to undermine attempts to end civil conflict.

Since Abiy came to power in 2018 after nearly three decades of firm rule by a TPLF-led government, political reforms have encouraged regional powerbrokers to seek to build ethnic power bases.

The government announced a military crackdown in the region after around 340 people were killed there last month.

The EHRC called for government security forces in Oromiya to be reinforced in light of the latest attacks.

The region is home to the Oromo, the country's largest ethnic group, whose members include Abiy. Members of other communities, including the Amhara, Ethiopia's second biggest ethnic group, also live there.

The EHRC said that the villages caught up in Monday's violence were mainly populated by Amhara, and that government security forces had reached the area.