JISR AL-SHUGHUR, June 25 (AFP) - Russian air strikes killed at least 13 people in rebel-held northwest Syria on Sunday (Jun 25), the deadliest attack on the country this year, a war monitor said.

At least nine civilians, including two children, were among the dead, with most killed at a fruit and vegetable market in Jisr al-Shugur in the Idlib region.

"These Russian strikes are the deadliest in Syria this year and amount to a massacre," said Rami Abdel Rahman, who heads the Britain-based Syrian Observatory for Human Rights.

Russian forces which back President Bashar al-Assad's regime were responding to rebel drone strikes over the past week that killed four civilians including two children, according to the Observatory.

Saad Fato, 35, a labourer who survived the strike on the market, told AFP he helped efforts to try to rescue the wounded.

"Russians shells rained on us," he said, recounting that he was unloading tomatoes and cucumbers at the time of the attack.

"It was indescribable, seeing the dead, the wounded," he said, his hands still covered with their blood.

An AFP correspondent at the scene saw plumes of black smoke rising from the site and ambulances, their sirens wailing, rushing the wounded from the market to hospital.

The Syrian Defence Ministry said in a statement on Sunday evening that its armed forces had "cooperated" with the Russian air force in retaliation for attacks over the past few days that killed civilians in Hama and Latakia provinces.

The operation targeted "terrorist positions" in Idlib province, killing dozens and destroying arms depots and drones, said the statement carried by Syria's official news agency SANA.