The attack took place in Azaz, a city that plays a crucial role in Turkey's operations in Syria that aims to secure a safe zone for Syrian refugees.
The death from Sunday’s car bombing in northwestern Syria has risen to 21, the White Helmets civil defence agency said Monday.
At least 45 people were also injured during the explosion, the White Helmets added.
Four children were among those killed in the explosion in Azaz, a city that plays a critical role in Turkey's military operations in Syria, said the Syrian Observatory for Human Rights.
"Many people were leaving evening prayers when the explosion happened," Observatory head Rami Abdul Rahman told AFP.
The incident took place at a market in the opposition-held city of Azaz, near the Turkish border.
No group has yet claimed responsibility for the attack.
The bombing comes one day after a similar explosion killed 10 people and wounded 20 in the northeastern city of Raqqa.
Since 2016, Turkey’s Euphrates Shield and Olive Branch operations in northern Syria have liberated the region from the YPG and DAESH terrorists, including Al Bab, Afrin, and Azaz, making it possible for Syrians who fled the violence there to return home.
The YPG is its Syrian branch of the PKK terrorist organisation.
In its more than 30-year terror campaign against Turkey, the PKK - listed as a terrorist organisation by Turkey, the US and the EU - has been responsible for the deaths of some 40,000 people, including women and children.
Although the PKK is listed as a terrorist organisation by both Turkey and the US, the US considers the group's Syrian branch to be an ally in the fight against Daesh.
Turkey, on the other hand, treats the YPG and Daesh as one and the same, and has declared its intention to clear both groups from its border with Syria.
Ankara keeps Turkish troops in the area and backs the local police forces to maintain security.
It has been eight years since the devastating conflict began to tear apart the country in 2011 when the Bashar al Assad regime cracked down on demonstrations with unexpected ferocity.
Thousands of people have been killed since then while millions of Syrians fled the country to seek refuge in neighbouring countries and beyond.