The Syrian Observatory for Human Rights, a monitoring organization based in Britain, said more than 50 people died including over 30 civilians. Two rebels contacted by Reuters 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.
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