At least 46 people, including 11 children, were killed when a bomb in a fuel truck exploded in the city of Afrin, which has been under the control of Turkish-backed fighters for two years.
Turkey blamed the attack on the Kurdish YPG group, which says it does not target civilians.
The United Nations secretary-general again appealed for an immediate ceasefire in Syria following what he described as the "horrific" bombing of a market in the north of the country.
On Wednesday, UN officials warned that with COVID-19 looming, there is an urgent need to end violence and ensure the delivery of critical medical supplies to Syrians.
Al Jazeera's diplomatic editor James Bays reports.
