Story highlights More than 20 children were among the dead in an attack on a school compound

The US and France said either the Syrian regime or its Russian backers carried it out

Washington (CNN) The US and Russia are exchanging ever harsher rhetorical barbs amid growing charges of war crimes in Syria following a brutal attack on a Syrian school.

More than 20 children were among the dead in the attack on the school compound in the rebel-held village of Hass in Idlib province Wednesday.

UN Secretary General Ban Ki-moon said he was "appalled" at repeated bombings of the school compound.

"If deliberate, this attack may amount to a war crime," he said in a statement.

Idlib, a main opposition stronghold is regularly hit by both Syrian and Russian airstrikes, as well as the US-led collation hitting ISIS targets.

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