Gunmen have killed at least 32 people and set fire to homes in a "horrendous" attack in western Ethiopia, officials say.

Local authorities said the rebel Oromo Liberation Army (OLA) was to blame for the attack in Oromia state.

Residents said dozens were rounded up and killed and livestock was stolen.

Prime Minister Abiy Ahmed suggested the attacks may have been identity-based. Ethnic violence has increased since he took office in April 2018.

The OLA is an armed group that has been blamed for kidnappings and bomb attacks in western and southern Ethiopia. The OLA broke off from the Oromo Liberation Front (OLF) - an opposition party that spent years in exile and returned to the country after Mr Abiy took office in 2018.

A local official told the BBC a team had been sent to Guliso district - the location of the violence - to investigate, anticipating the death toll could be "high".

The Ethiopian Human Rights Commission (EHRC) said up to 60 "armed and unarmed assailants" were involved in the attack, which was carried out on Sunday.

It said members of the Amhara ethnic group, the country's second largest ethnic group, were targeted. The Amhara branch of the ruling Prosperity Party also released a statement supporting the EHRC's report.

A survivor told AFP news agency that security forces stationed in the area left and the OLA then rounded up civilians.

"After collecting us, they opened fire on us, and then afterwards looted cattle and burnt down houses," they said.

Mr Abiy Ahmed later said that "measures have started to be taken against the attackers".

"I am deeply saddened by the ongoing identity-based attacks on Ethiopians," he wrote on Twitter.

In June, a wave of ethnic unrest broke out following the killing of popular Oromo singer Hachalu Hundessa, leaving more than 150 people dead.