Fighting has ravaged large tracts of Myanmar since a military coup in 2021 that sparked renewed clashes with ethnic rebel groups, as well as the formation of dozens of "People's Defence Forces" now battling the junta.

The military bombed a gathering in an opposition stronghold last month that media and locals said killed about 170 people, sparking renewed global condemnation.

The military had used a thermobaric bomb in the attack on Pazi Gyi village in Sagaing region, HRW said. Their assessment was based on analysis of 59 photos of the victims' bodies and a video of the site following the strike.

Thermobaric bombs, also known "vacuum bombs" are more powerful than conventional high-explosive munitions and use two separate charges.

The first scatters a fuel mixture as a cloud around the target and the second detonates the cloud, sucking the oxygen out of the atmosphere and forming a huge fireball.

The thermobaric bomb used in Pazi Gyi had caused "indiscriminate and disproportionate civilian casualties... and was an apparent war crime," HRW said.

Thermobaric weapons are not specifically banned under international conventions but their use against civilian populations could result in war crimes convictions.

The junta said it had launched "limited air strikes" on Pazi Gyi but said most of the dead had been killed after the strikes hit an ammunition store.

Myanmar is set to dominate a meeting of leaders from the Association of Southeast Asian Nations (ASEAN) that started in Indonesia on Tuesday.

The bloc has led diplomatic efforts to resolve the bloody crisis but the junta, backed by allies and arms suppliers Russia and China, has refused to negotiate with its opponents.

HRW called on the United Nations Security Council to adopt an arms embargo on Myanmar, refer the junta to the International Criminal Court and impose sanctions on the military's sprawling business empire.

"The Myanmar junta's abusive military operations depend on its ability to purchase weapons and materiel," HRW said.

"ASEAN and the UN Security Council both need to reconsider their toothless approaches to Myanmar's junta and take stronger measures."