At least 11 schoolchildren died in an air strike and firing on a Myanmar village, the UN children's agency said, an attack the country's junta said targeted rebels hiding in the area.
Myanmar has been in chaos since the military seized power in a coup in February last year, with nearly 2,300 civilians killed in a crackdown on dissent according to a local monitoring group.
The Sagaing region in the country's northwest has seen some of the fiercest fighting, and amid clashes between anti-coup fighters and the military, entire villages have been burned down.
The UN children's agency UNICEF condemned Friday's incident in Depeyin township in Sagaing.
"On 16 September, at least 11 children died in an air strike and indiscriminate fire in civilian areas," UNICEF said in a statement issued Monday.
It said schools must be safe and never targeted.
"At least 15 children from the same school are still missing," UNICEF said, calling for their immediate safe release.
Video footage obtained from a local community group shows a classroom with blood on the floor, damage to the roof and a mother crying over her son's dead body.
The junta confirmed they had sent troops in helicopters to the village after receiving a tip-off that fighters from the Kachin Independence Army -- an ethnic rebel group -- and from a local anti-coup militia were moving weapons in the area.
The military accused the rebel fighters of using civilians as human shields, and said it had seized mines and explosives from the village.
Save the Children Asia Regional Director Hassan Noor expressed condolences to the families and said schools should be off limits and the safety of students protected.
"How many more incidents like this need to take place before action is taken?" Noor said, urging the UN Security Council and Association of South East Asian Nations (ASEAN) to take swift action.
ASEAN has led so far fruitless diplomatic efforts to resolve the crisis in Myanmar and its leaders will meet in Phnom Penh in November to discuss limited progress on a peace plan.