Syrian Observatory for Human Rights says those killed in two attacks include 14 children and four women.
Air raids carried out by Syrian regime and Russian warplanes in Syria's western province of Idlib killed at least 39 civilians on Sunday.
A local civil defence official Leys al-Faris said a Syrian regime's warplane launched an air strike on a market in the rebel-held province.
At least 36 people were killed and 50 others injured in the attack, he said, describing it as a "massacre" perpetrated by the Bashar-al-Assad regime during the holy month of Ramadan.
Three more civilians were killed in a separate air strike carried out by a Russian plane on a residential area in Maarret al-Numan, another town of Idlib province.
"Rescuers are looking for survivors following the attack," al-Faris said.
According to the British-based Syrian Observatory for Human Rights, those killed in the two attacks, in the Idlib province, include 14 children and four women.
Footage shared by the Observatory, claimed to have been shot after the Idlib strikes, showed emergency workers training water hoses on a tall building amid a haze of smoke.
Syrian regime forces and Russian warplanes have intensified attacks on rebel-controlled areas in Syria since April 21, killing dozens of civilians.
Suspected regime strikes killed at least 37 civilians in Maaret al-Numan in April, sparking condemnation from Syria's opposition amid faltering peace talks.
Russia deployed warplanes to Syria last year to support the Bashar al-Assad regime against rebels seeking to end his rule.
Soon after the attacks some horrific images and videos were shared on social media.
In such a horrible day, this is what gives us hope and keeps us going. SCD rescues a baby in Ma'rat Al Noman -Idlib.https://t.co/ObGX7J26zJ— The White Helmets (@SyriaCivilDef) June 12, 2016
1 child killed, 6 children & their mother still under rubble in Ma'rat Al Noman as an Airstrike targeted a market. pic.twitter.com/XUQSqe4IFK— The White Helmets (@SyriaCivilDef) June 12, 2016
280,000 killed in five years
Peace talks aimed at ending the five-year civil war in Syria have deadlocked and there is no immediate end in sight to a conflict that has so far claimed 280,000 lives.
Syria has been locked in a vicious civil war since early 2011, when the Assad regime cracked down on pro-democracy protests with unexpected ferocity.
More than a quarter of a million people have been killed and more than 10 million have been displaced across the war-battered country, according to the United Nations.