"Six civilians were killed in Jisr al-Shughur and three rebel fighters were killed nearby by Russian air strikes," Rami Abdel Rahman, who heads the Syrian Observatory for Human Rights, told AFP.

Ahmed Yezidi of the civil defence in Jisr al-Shughur, a city in rebel-held Idlib province, said the strikes killed nine people, without specifying whether fighters were included in the toll.

A fruit and vegetable market in the city was hit by the Russian strike, said the Observatory and an AFP correspondent at the scene.

Yezidi called it "a direct attack on the popular market, which is a basic source of income for farmers" in the area.

One civilian and one rebel fighter were also killed in a strike on the outskirts of Idlib city, said Abdel Rahman, whose Britain-based monitor has a wide network of sources inside war-torn Syria.

At least 30 civilians were wounded in Sunday's strikes, he said, adding that the death toll was likely to rise.

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

Damascus, with Russian and Iranian support, has clawed back much of the ground lost in the early stages of Syria's conflict, which erupted in 2011 when the government brutally repressed pro-democracy protests.

The last pocket of armed opposition to the regime includes large swathes of Idlib province and parts of the neighbouring Aleppo, Hama and Latakia provinces.

Hayat Tahrir al-Sham, headed by ex-members of Syria's former Al-Qaeda franchise, is the dominant group in the area but other rebel groups are also active, with varying degrees of Turkish backing.

Syria's war has killed more than half a million people and forced around half of the country's pre-war population from their homes.