Bombs dropped by suspected Russian warplanes killed at least 12 Syrian schoolchildren on Monday when they hit a classroom in a rebel-held town in Aleppo province, the Syrian Observatory for Human Rights reported.
The air strike hit the town of Injara some 15 km (9 miles) west of Aleppo city. A teacher also died and there were reports of others wounded, some critically, the monitor said.
Social media footage released by opposition activists showed a classroom with destroyed benches and textbooks lying on the floor stained with blood. The footage could not be independently verified.
There was no immediate from the Russian defense ministry.
In Geneva, a spokesman for the UN children's fund UNICEF said it was looking into the reports of the raid.
The Kremlin launched air strikes over Syria in September saying it wanted to help President Bashar al-Assad, its main Middle East ally, defeat Islamic State and other militant groups.
Rescue workers and rights groups say the bombings have killed scores of civilians at busy market places and in residential areas away from the frontlines. Russia denies this.
Amnesty International said last month that Moscow's actions had violated humanitarian law. US officials say Russia used fewer precision-guided munitions than the United States and its allies.