Children were among nine people killed in an air strike on a high school in a rebel-held city of northern Syria on Sunday, the Syrian Observatory for Human Rights said.

"The Syrian air force bombed a technical high school in the city of Raqa, killing nine people, among them children under 18, and wounding several other people," the Britain-based watchdog said.

Raqa, on the Euphrates valley 160 kilometres (100 miles) east of the main northern city of Aleppo, is the only provincial capital entirely in rebel hands.