At least 32 Syrian rebels were killed in a suicide attack late Sunday at a border crossing with Turkey, a monitoring group said.
A suicide bomber detonated an explosives belt inside a bus carrying rebels from Turkey into Syria through the Atmeh border crossing some 40 kilometres west of Aleppo, the Syrian Observatory for Human Rights said.
There were conflicting reports on the exact location of the attack. The Britain-based monitoring group said it took place on the Turkish side of the crossing and at least two Turkish border guards were also killed.
Turkish news agency DHA reported that the blast took place on the Syrian side and did not mention any casualties among Turkish personnel.
Syrian opposition outlet Orient News, quoting local activists, said the attack occurred outside a refugee camp on the Syrian side of the border.
The rebels hit in the blast were rotating back into Syria to replace fighters returning from action, the Observatory said.
There was no immediate claim of responsibility for the attack. The Islamic State extremist group has repeatedly carried out suicide attacks on Syrian rebel groups.
Since the Syrian uprising started in 2011 the Syrian government has accused Turkey of using its border to facilitate the flow of rebel fighters into Syria.
The pro-opposition Observatory, which is based in Britain, has previously reported transfers of rebels from western Aleppo province and neighbouring Idlib province to rebel-held areas north of Aleppo via Turkish territory.
Rebels from Jaish al-Fath, a coalition dominated by hardline Islamist group Ahrar al-Sham and former al-Qaeda affiliate Jabhat Fatah al-Sham, control most of the territory on the Syrian side of the border.