Air strikes that hit a school in Syria's rebel-held Idlib province killed 22 children and six teachers, the UN's children agency UNICEF said.

"This is a tragedy. It is an outrage. And if deliberate, it is a war crime," said UNICEF director Anthony Lake.

The air strikes were carried out by Syrian or Russian warplanes, rescue workers and a monitoring group said.

The raids hit a residential area and a school in Haas village, the Syrian Civil Defence rescue workers' network said on its Facebook account.

A report on Syrian state TV quoted a military source saying a number of militants had been killed when their positions were targeted in Haas, but made no mention of a school.

Mr Lake said the school compound was "repeatedly attacked," adding that it may be the deadliest attack on a school since the war began more than five years ago.

"When will the world's revulsion at such barbarity be matched by insistence that this must stop?" he asked.