"There was (a People's Defence Force) office opening ceremony ... (Tuesday) morning about 8 am at Pazi Gyi village," junta spokesman Zaw Min Tun confirmed late Tuesday night.

He said some of the dead were anti-coup fighters in uniform while acknowledging "there could be some people with civilian clothes."

"According to the ground information we got, people were killed not because of our attack only. There were some mines planted by PDF around that area," he said, adding the airstrike had also hit a storage area for gunpowder and mines.

UN rights chief Volker Turk said Tuesday 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 in the remote Kanbalu township in Sagaing region remains unclear.

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

Before military aircraft strafed Pazi Gyi village, scores of locals had gathered to mark the opening of a local People's Defence Force office.

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.