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

The Syrian Observatory for Human Rights said 'warplanes - either Russia or Syrian - carried out six strikes' in the village of Hass, including on a school complex, killing 11 schoolchildren.

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.

A photograph circulated on social media showed a child's arm, seared off above the elbow, still clutching the strap of a dusty black rucksack.

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

Asked about the attack, Russian Ambassador Vitaly Churkin responded: "It's horrible, horrible. I hope we were not involved."

"It's easy for me to say 'no' but I'm a responsible person. I need to see what our minister of defense is going to say," he told reporters.

Syrian government forces and their Russian ally have been accused by Western powers and rights groups of carrying out indiscriminate attacks on civilian infrastructure.

More than 300,000 people have been killed in Syria and over half of the country's population displaced since the conflict began in March 2011 with anti-government protests.

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