The Islamic State militants group (IS) on Monday claimed responsibility for a suicide bombing that killed more than 30 Syrian rebel fighters in a blast near the Turkish border.
IS, in a statement published on Twitter and the messaging service Telegram, said a suicide bomber detonated an explosive belt among rebels "getting ready to fight the Islamic State".
The blast on Sunday night struck the Atme border crossing between Turkey and the northern Syrian province of Idlib, according to the Syrian Observatory for Human Rights.
The monitoring group said the anti-regime fighters were travelling by bus to Turkey then back into Syria towards the rebel bastion of Azaz, in Aleppo province.
The attack killed 32 rebels as they were entering Turkey, the Britain-based Observatory said, updating an earlier toll on Sunday evening of 15.
Its head Rami Abdel Rahman said the toll could rise further as other fighters remained critically wounded.
IS's statement said the attack left "nearly 50" rebels dead, charging they took orders from the US-led coalition fighting IS in Iraq and Syria.
IS has tried to advance on rebel territory in Aleppo province -- particularly the towns of Azaz and nearby Marea -- for several months.
A major militant offensive in May saw IS encircle Marea, but rebels pushed back the assault the following month after receiving ammunition from the coalition.
More than 290,000 people have been killed since Syria's conflict erupted in March 2011.