The investigators pointed to multiple reports indicating that the school, located in the compound of a monastery, came under attack for several hours, first from helicopters firing rockets and machine guns, followed by an infantry attack.

The United Nations' Independent Investigative Mechanism for Myanmar (IIMM) said in a statement that the September 16 attack in Let Yet Kone village in the northwestern Sagaing region "may be considered a war crime with commanders criminally liable."

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 IIMM, which was established by the UN Human Rights Council in September 2018 to collect evidence of the most serious international crimes, said it was working to "assess criminal responsibility" for the attack.

"Armed attacks that target civilians are prohibited by international laws of war and can be punished as war crimes or crimes against humanity," the statement said.

Teachers from the school told AFP that some children were playing outside while others attended classes as two helicopter gunships flew in and opened fire with machine guns and heavy weapons.

They described the terrifying horror of the onslaught, including watching a wounded child screaming in agony, begging to die.

The Myanmar junta said they had sent troops in helicopters to the village after receiving a tip-off that fighters from the Kachin Independence Army (KIA) -- an ethnic rebel group -- and from a local anti-coup militia were moving weapons in the area.

The military accused the rebels of using civilians as human shields, and said it had seized mines and explosives from the village.

IIMM warned though that "even if this was the case and the armed attack had a military objective, it is prohibited according to the laws of war if it is expected to result in civilian injuries or deaths that are excessive in relation to the expected direct military advantage achieved by the attack."

The investigators stressed that commanders who launch a military attack near civilians have obligations under international law, including taking all possible precautions to minimise harm to non-combatants.