The UN has expressed deep concern about an escalation of hostilities in north-western Syria after at least 11 people were reportedly killed in Russian air strikes on rebel-held Idlib province.

Nine were killed when warplanes dropped bombs next to a market outside Jisr al-Shughour on Sunday, rescuers said.

The Syrian Observatory for Human Rights monitoring group reported that it was the deadliest Russian attack this year.

Another two people were killed in an air strike near Idlib city, it said.

The Syrian defence ministry said its troops had co-operated with the Russian air force in an operation that targeted "terrorist headquarters and warehouses" in Idlib and "eliminated dozens of terrorists".

The strikes were retaliation for attacks that had killed civilians in government-held Hama and Latakia provinces, it added.

The White Helmets, whose first responders operate in opposition-held areas, said most of those killed near Jisr al-Shughour were workers and farmers at a vegetable market that was next to the site targeted in the Russian strike.

The UN's special envoy for Syria, Geir Pedersen, said he was "deeply concerned" by the escalation of violence and called for an immediate halt to all attacks on civilians.

"The people of Syria have suffered enough," he said in a statement. "They need peace, not more violence."

The latest violence comes as the Syrian government is preparing for a major offensive to retake Idlib province, the last remaining opposition stronghold.

The UN has warned that an all-out offensive on Idlib could lead to a humanitarian catastrophe, with millions of people displaced and thousands of civilians killed.

The Syrian conflict has killed more than half a million people and displaced millions more.