Russian missiles
Russian missilesiStock

Russian air strikes killed at least 13 people in rebel-held northwest Syria on Sunday, the deadliest attack on the country this year, a war monitor said, according to AFP.

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.

The Syrian Defense 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.

Six civilians died in Jisr al-Shughur and three rebel fighters were killed nearby by Russian air strikes, said Abdel Rahman, whose group relies on a network of sources inside Syria.

Another three civilians, including two children, and one rebel fighter were killed in a strike on the outskirts of Idlib city, said Abdel Rahman.

At least 30 civilians were wounded in Sunday's strikes, said Abdel Rahman, who added that the death toll was likely to rise.

Russia has been conducting air strikes in Syria since September 2015, in support of Syrian President Bashar Al-Assad, its close ally.

While Russia said it was attacking the Islamic State (ISIS), Western officials claimed Moscow was also targeting so-called "moderate" rebel groups fighting ISIS.

The Assad regime, with Russian and Iranian support, has clawed back much of the ground lost early in Syria's conflict which erupted in 2011.

Russia has over the years repeatedly struck Syria's last pocket of armed opposition to the regime in the northwest.