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 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.
- 'Direct attack' -
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.
That fighter was a member of the Turkistan Islamic Party, a Uyghur-dominated jihadist group, he said, adding that the parents of the dead children also belonged to that group.
At least 30 civilians were wounded in Sunday's strikes, said Abdel Rahman, who added that the death toll was likely to rise.
Ahmed Yazigi of the civil defence in Jisr al-Shughur earlier reported that the strikes killed nine people, without specifying whether the toll included fighters.
Yazigi called the assault "a direct attack on the popular market which provides a basic source of income for farmers".
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.
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 when it brutally repressed pro-democracy protests.
Russia has over the years repeatedly struck Syria's last pocket of armed opposition to the regime in the northwest.
But deadly attacks on civilians had been limited so far this year until the latest Russian strikes.
On Saturday, a Russian airstrike killed two civilians in the Idlib region.
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