The Southeast Asian country has been in chaos and its economy in tatters since the military seized power in February 2021 coup.

UN rights chief Volker Turk said he was "horrified" by the deadly air strikes whose victims he said included schoolchildren performing dances, with the global body calling for those responsible to be brought to justice.

The death toll from the early Tuesday morning strike on the remote Kanbalu township in Sagaing region is unclear.

At least 50 fatalities and dozens of injuries were reported by BBC Burmese, The Irrawaddy and Radio Free Asia.

The United Nations, while not confirming a toll, said several civilians were killed.

"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," Turk said in a statement from Geneva, accusing Myanmar's military of once again disregarding "clear legal obligations... to protect civilians in the conduct of hostilities".

UN Secretary-General Antonio Guterres "strongly condemns the attack by the Myanmar Armed Forces today (and) calls for those responsible to be held accountable," according to a statement by his spokesman Stephane Dujarric.

Guterres "reiterates his call for the military to end the campaign of violence against the Myanmar population throughout the country," the spokesman said.

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

Graphic video clips circulating on social media -- footage AFP has been unable to verify -- show bodies scattered among ruined homes.

"We are going to rescue you if we hear you screaming," one person could be heard saying in the video. "Please scream!"

A rescuer connected to an anti-coup People's Defence Force group told AFP that women and children were among the dead.

After recovering bodies and transporting victims for medical treatment, he estimated the death toll could be up to 100.

- 'Great pain' -

Before military aircraft strafed Pazi Gyi village, scores of locals had gathered to mark the opening of a local defence force office.

Myanmar's National Unity Government (NUG), a shadow body dominated by former lawmakers from ousted civilian leader Aung San Suu Kyi's party, condemned the strike as a "heinous act".

"We ... share the great pain felt by the families affected by this tragedy," it said in a statement.

The military, which accuses anti-coup fighters of being terrorists, has faced international condemnation for razing villages, mass killings and air strikes on civilians.

More than 30 people sheltering in a monastery were killed in Shan state in March.

Last year, a military air strike on a concert put on by the Kachin Independence Army in northern Kachin state killed around 50 people and wounded more than 70, the rebels said.

At a military parade last month, junta leader Min Aung Hlaing vowed to continue cracking down on opponents.

The military last month announced a six-month extension of a state of emergency and postponed elections it had promised to hold by August because it did not control enough of the country for a vote.

More than 3,200 civilians have been killed and more than 21,300 arrested since the coup, according to a local monitoring group.