More than 60 people were also injured in the airstrikes, Al-Khatib said, adding that he fears the death toll may rise.
Three strikes hit the northern town of Atareb on Monday, the Syrian Observatory for Human Rights said, despite a "de-escalation zone" in place there.
Head of the monitor Rami Abdul Rahman said that a number of people died from injuries they sustained during Monday's strikes, bringing the toll to 61, including five children.
The town is located in an area of Aleppo province that is part of a "de-escalation zone" established earlier this year by Russian Federation and Iran - which support the Syrian government - and Turkey, which backs the rebels.
The Observatory said it could not determine whether Russian Federation or the Syrian government was behind the attack.
Three airstrikes targeted the marketplace, where approximately 100 people were present in the opposition-held town of Atarib, Khalid Al-Khatib, an official of the pro-opposition Syrian civil defense teams (White Helmets), told Anadolu Agency.
Russia, a key ally of Syria's President Bashar al-Assad, began a bombing campaign in support of his government in 2015.