UNICEF says at least 11 children have been killed in a helicopter attack on a school in northwestern Myanmar. The country's pro-democracy shadow government believes the assault was carried out by the military.

UNICEF Myanmar says the airstrike took place on Friday in civilian areas of Sagaing Region. It says at least 15 children from the school are still missing, and further details are being clarified. It condemns the attack, saying schools must be safe and children must never be attacked.

The pro-democracy National Unity Government said in a statement soldiers detained about 20 children and teachers after the strike.

The military says it exchanged fire after being attacked by an armed group.

A man whose niece was killed spoke to NHK. He said, "I didn't see her body. They took it immediately. Nobody was allowed to enter the classroom. Soldiers picked up the pieces of the dead bodies and put them into bags, and put them on the truck."

Children have been caught up in the conflict since the military took power in a coup last year. In June, the UN special rapporteur on human rights in Myanmar said more than 250,000 have been displaced, and over 380 have been killed or seriously injured.