Suicide bomber targets checkpoint outside key city after it was captured by Turkey-backed rebels a day earlier.
A car bomb in a village near al-Bab struck Syrian rebels fighting Islamic State of Iraq and the Levant (ISIL), killing at least 45 people and wounding dozens more, medical sources said.
The suicide bomber targeted a checkpoint manned by Free Syrian Army rebels that was crowded with civilians early Friday in the village of Sousian, northwest of al-Bab.
Turkey-backed rebels on Thursday drove ISIL from al-Bab, the group’s last significant stronghold in northwest Syria, along with two smaller neighbouring towns of Qabasin and al-Bezah after weeks of street fighting.
Al Jazeera’s Andrew Simmons, reporting from Gaziantep on the Syria-Turkey border, said the attack took place about 10km outside of al-Bab.
The medical charity Ambulanciers Sans Frontieres said 45 people, mostly civilians, had been killed and about 70 others were wounded.
"A large number of civilians were gathering around a checkpoint and suddenly a suicide car bomber drove in and detonated," Simmons said.
"There could be Free Syrian Army members counted in the casualty figures as well. This is really a warning shot from ISIL it would appear."
The car bomb hit outside a security office where civilians had gathered seeking permission to return to al-Bab.
A rebel fighter with the Sultan Murad Brigade near al-Bab told Reuters news agency: "It was done on a checkpoint but there were a lot of families there gathered and waiting to get back to al-Bab. Therefore, we have many civilian casualties."
ISIL said in a social media posting that it was behind the attack.
"There are still lots of cells inside Bab. It is very dangerous. Our search-and-clear operation is still under way," the rebel fighter said.
On Thursday, several Turkey-backed rebels were killed by a mine in al-Bab while clearing the town of unexploded ordnance after ISIL retreated, the group said.
Syria’s main conflict pits President Bashar al-Assad, backed by Russia, Iran and Shia militias, against rebels that include groups supported by Turkey, the United States and Gulf monarchies.
However, both those sides, as well as a group of militias led by Kurdish forces and supported by the US, are also fighting ISIL, which holds large swathes of northern and eastern Syria.
Turkey directly intervened in Syria in August in support of a group of rebel factions fighting under the Free Syrian Army banner to drive ISIL from its border. It also wants to stop Kurdish groups from gaining control of most of the frontier.