Dozens of people have been killed in government airstrikes during celebrations at a concert in Myanmar, rebel groups said.

Ethnic Kachin groups said at least 30 people, including singers and musicians, were killed in Myanmar military strikes.

They were attending an anniversary celebration of the minority’s main political organization, Kachin Independence, in a remote mountainous region about 600 miles from the country’s largest city.

The Kachin Artists Association said between 300 and 500 people attended the event, and the dead included a Kachin singer and a keyboard player.

Also among those killed were Kachin officers and soldiers, musicians and at least 10 Kachin military and businessmen seated in front of the stage.

The death toll during Sunday night’s celebrations was the heaviest in a single airstrike since the military seized power in February.

The United Nations said it was “deeply concerned” and “saddened” by the reports, adding that initial estimates suggested more than 100 civilians had been affected.

It described the attack as “excessive and disproportionate use of force by security forces against unarmed civilians.”

“We extend our deepest condolences to the families and friends of all those killed or injured. The UN calls for immediate medical assistance for the victims, if necessary,” the statement said.

The incident comes just three days before Southeast Asian foreign ministers are due to meet in Indonesia to discuss rising violence in the country.

Myanmar has been wracked for decades by insurgencies by ethnic minorities seeking autonomy, but anti-government resistance has grown markedly across the country since the creation of an armed pro-democracy movement that opposed last year’s military takeover