A double suicide car bombing at the Bab al-Hawa border post between Syria and Turkey on Monday killed at least 16 people, including six militants.
The opposition observatory, updating an earlier toll, said 20 people were wounded as one car detonated at a checkpoint just outside the crossing and another inside the post.
The border crossing in Idlib governorate of northwest Syria is held by Islamist and opposition militants who have been locked in fierce fighting with jihadists from the Islamic State of Iraq and the Levant in recent weeks.
ISIL fighters on Monday took control of the Jarrah military airport in the northern governorate of Aleppo that had been in opposition hands since February 2013.
Islamist militants angered by ISIL's alleged abuses have forced them from Aleppo city but the group has managed to strengthen its grip on the town of Raqa, also in northern Syria.
ISIL has alienated activists and other opposition fighters in Syria, who accuse it of imposing a reign of terror on areas where it operates.
Its actions provoked a backlash from powerful groups in early January, and the group is now fighting opposition forces in several parts of northern Syria.
The Observatory said Monday that ISIL fighters "withdrew after midnight yesterday (Sunday) from the western Aleppo countryside and evacuated their positions".
The Britain-based observatory also reported heavy clashes between jihadists and militants on the outskirts of the town of Azaz in Aleppo province, near the border with Turkey.
Militant groups in the town shelled areas where ISIL fighters were with mortar rounds, the Observatory said.