They included 13 people who were killed when warplanes hit a public market in the town of Maaret al-Noumaan, in the southern countryside of Idlib.
Some 18 others were wounded, most of whom were in serious condition, according to the Observatory.
Two others were killed in other areas in the province, it added.
Rami Abdel Rahman, the head of the Observatory, said that Russian planes struck areas in the village of Kfar Sijna, south of Idlib city.
Meanwhile, shelling by Turkish forces and their Syrian rebel allies on an area controlled by Kurdish militias in northern Syria has killed eight children and one man, the war monitor reported.
The shells hit near a school in Tal Rifaat, in northern Aleppo, where there are Syrian government troops deployed as well as a Russian base.
The Observatory said that 21 people were injured in the shelling.
A source in the Kurdish-led Syrian Democratic Forces (SDF) confirmed the shelling and said most of the wounded had serious injuries.
Kurdish activists posted pictures of young children whose bodies had been ripped apart.
Tel Rifaat is home to many Kurdish refugees who escaped Afrin, a north-eastern Syrian town which was seized by Turkish forces and their rebel allies in March 2018.
According to the Observatory, some 4,775 people have lost their lives, among them 1,224 civilians, since the Syrian government began a massive military offensive in April against rebels in the provinces of Hama and Idlib. Thousands have been forced to flee.
Government forces have since regained 66 areas in the southern section of Idlib and the northern countryside of Hama from rebels.
Over the past 48 hours, at least 96 people - 51 government forces and 45 rebels - have died in Idlib during heavy battles. The Observatory said government forces, supported by Russian and Syrian fighter jets, clashed with rebel groups on several fronts in the south-eastern countryside of Idlib over the weekend.
The watchdog said it was the heaviest loss of life since a Russian-brokered ceasefire came into effect in late August.
Activists based in Idlib said the battles and the heavy airstrikes by Syrian government forces led to a new wave of displacement from the south-eastern areas of the province towards areas near the border between Syria and Turkey.
They added that areas in the south-eastern countryside have been subjected to hundreds of airstrikes since Friday.
In a separate incident, a building that was the target of heavy shelling in previous years collapsed in the government-held al-Maadi neighbourhood of Aleppo, killing nine people and wounding 21 others.
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