"It appears schoolchildren performing dances, as well as other civilians, attending an opening ceremony at the hall in Pazi Gyi village, Kanbalu Township, were among the victims," Volker Turk said.

Describing how "a helicopter gunship then reportedly fired on those fleeing the hall", he pointed to reports of up to 100 people killed in the strikes early Tuesday morning in the Sagaing region, an opposition stronghold.

Witnesses contacted by AFP also spoke of as many as 100 killed, while graphic video clips circulating on social media -- that AFP has been unable to verify -- show bodies scattered among the ruins of homes.

Turk accused Myanmar's military of once again disregarding "clear legal obligations... to protect civilians in the conduct of hostilities" and showing "blatant disregard for the related rules of international law".

"I also call on all parties to take all feasible precautions to protect the civilian population that is under their control from the effects of attacks, including by avoiding locating military objectives within or near densely populated areas," he said.

Myanmar has been in chaos and its economy in tatters since the military seized power in a coup two years ago.

Sagaing region -- near the second-largest city Mandalay -- has put up some of the fiercest resistance to the military's rule, with intense fighting raging there for months.

Turk reiterated that "there are reasonable grounds to believe that the military and its affiliated militias are responsible for an extremely broad range of human rights violations and abuses since 1 February 2021".

Some of those violations, he warned, "may constitute crimes against humanity and war crimes".

"I firmly believe the international justice processes now underway will one day hold the military leadership accountable for such crimes," he said.