Children's charity Unicef has said the bombing of a school in Syria amounts to a war crime.
At least 20 pupils were killed in the attack on a school in Syria's rebel-held Idlib province earlier this week that has been blamed on Russia. Unicef said 22 children and six teachers were killed.
"This is a tragedy. It is an outrage. And if deliberate, it is a war crime," said UNICEF director Anthony Lake, who added the school compound had been struck repeatedly.
The air strikes were carried out by Syrian or Russian warplanes, rescue workers and a monitoring group said.
Moscow is supporting the Assad regime as it tries to crush the country's rebel forces.
US Secretary of State John Kerry has called for an investigation into the bombing.