The blast in the village of Sousian hit a security checkpoint controlled by rebels fighting under the Free Syrian Army (FSA) banner.
The Syrian Observatory for Human Rights, a monitoring organization based in Britain, said more than 50 people died including over 30 civilians. Two rebels put the total death toll at at least 40.
One of the two, a fighter with the Sultan Murad Brigade near al-Bab, said: “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.”
The Turkish-backed rebels drove Islamic State from the town of al-Bab on Thursday, following weeks of street battles near an area where Ankara wants to establish a safe zone for civilians.
Turkey’s military said on Friday that Syrian rebels had taken full control of all of al-Bab, and that work to clear mines and unexploded ordnance was under way.
“With al-Bab under control, the planned targets … have been achieved. In the aftermath, support will be provided to normalize life and for the local people to quickly return to their homes,” said Turkish military chief of staff Hulusi Akar.
Sousian is behind rebel lines about 8 km (5 miles) northwest of al-Bab, around which Ankara has long supported the formation of a security zone it says would help to stem a wave of migration via Turkey into Europe.
A second blast took place 2 km south of Sousian later on Friday, but it was unclear whether it was from a vehicle bomb or a planted device such as a mine. There were reports of casualties but no immediate details, the Observatory said.
Islamic State said in a social media posting that it was behind the Sousian attack, having acknowledged on Thursday it had lost control of al-Bab.
Syria’s main conflict pits President Bashar al-Assad, backed by Russia, Iran and Shi’ite 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 United States, are also fighting Islamic State, which holds large parts of northern and eastern Syria.
As mines laid in and around al-Bab claimed lives for a second day, the Sultan Murad Brigade fighter said many IS cells were still operating there.
“It is very dangerous. Our search and clear operation is still under way,” he said.
Two Turkish soldiers were killed on Friday while clearing mines in the town of Tadef south of al-Bab, the military said. On Thursday, several Turkish-backed rebels were killed by a mine in al-Bab, the Observatory said.
Turkey directly intervened in Syria in August in support of rebel factions under the FSA banner to drive Islamic State from its border. It also wants to stop Kurdish groups gaining control of the region.
After taking al-Bab on Thursday, Turkish forces shelled Islamic State in Tadef, the Observatory reported.
The area immediately to the south of Tadef is held by the Syrian army and its allies, which have in recent weeks pushed into Islamic State territory in that area from Aleppo and advanced toward the Euphrates river.
Further east, the Syrian Democratic Forces, Kurdish-led militias backed by the United States, have in recent weeks taken dozens of villages from Islamic State as they close in on the group’s Syrian capital of Raqqa.