The rights watchdog said three bombs were dropped on January 7 on a school sheltering displaced Tigrayans in the town of Dedebit "killing at least 57 civilians and wounding more than 42 others".

"The Ethiopian government should carry out a prompt, thorough, and impartial investigation of the apparent war crime and appropriately prosecute those responsible," HRW said in a statement.

There were mainly elderly people, women and children sleeping in plastic tents there, it said, adding that there was no evidence of military targets at the site.

The conflict in northern Ethiopia began in November 2020 when Prime Minister Abiy Ahmed sent troops to disarm and detain the region's dissident leadership.

Based on debris recovered by survivors and aid workers and the extent of damage and injuries caused, HRW said a type of MAM-L guided bomb delivered by a Turkish-made drone and other light aircraft was likely responsible.

"Using guided bombs without evidence of any military target indicates that this was an apparent war crime," said Laetitia Bader, Horn of Africa director for HRW.

Abiy has accused the Tigray People's Liberation Front (TPLF) of orchestrating attacks on federal army camps in the region and promised a swift campaign to oust them.

But fighting dragged on and spread, displacing hundreds of thousands and fuelling a severe humanitarian crisis.

The conflict has claimed thousands of lives, and both sides have been repeatedly accused of human rights violations and atrocities.

In March, UN human rights chief Michelle Bachelet reported that at least 304 civilians were killed and 373 injured between late November and late February in aerial bombardments apparently carried out by the Ethiopian military.