UN rights chief Volker Turk said he was "horrified" by the deadly air strike, 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 the central Sagaing region remains unclear.

At least 50 fatalities and dozens of injuries were reported by BBC Burmese, The Irrawaddy and Radio Free Asia, as well as a witness contacted by AFP.

The junta confirmed late Tuesday night that the strike had taken place, but did not say how many were killed.

"There was (a People's Defence Force) office opening ceremony... (Tuesday) morning about 8 am at Pazi Gyi village," said spokesman Zaw Min Tun, referring to the armed anti-junta groups that have sprung up across the country since its elected government was toppled in a 2021 military coup.

"We attacked that place."

Some of the dead, he added, were anti-coup fighters in uniform, though "there could be some people with civilian clothes".

The spokesman went on to blame mines planted by the People's Defence Force for some of the deaths.

The United Nations, while not confirming a toll, said several civilians were killed, with Turk accusing Myanmar's military of once again disregarding "clear legal obligations... to protect civilians in the conduct of hostilities".

The military's crackdown on dissent in the wake of the coup has left more than 3,100 people dead, according to a local monitoring group.

UN Secretary-General Antonio Guterres "strongly condemns the attack by the Myanmar Armed Forces today", 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 added.

Washington said it was "deeply concerned" about the air strikes.

"These violent attacks further underscore the regime's disregard for human life and its responsibility for the dire political and humanitarian crisis in Burma following the February 2021 coup," State Department spokesman Vedant Patel said in a statement, using the country's former name.

"The United States calls on the Burma regime to cease the horrific violence, allow unhindered humanitarian access, and to respect the genuine and inclusive democratic aspirations of the people of Burma."

- 'Great pain' -

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 a 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 as high as 100.

Germany's Foreign Office in a tweet said it "strongly" condemned the strike, adding it expected "the regime to end the violence against its people immediately".

Myanmar's National Unity Government, a shadow body dominated by former lawmakers from ousted civilian leader Aung San Suu Kyi's party, called the strike 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 about 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.