Abu Abdullah al-Libi, a local chief of the group, was killed along with 12 other fighters from the jihadist organisation, the Syrian Observatory for Human Rights said.
"He was killed in clashes with a group of rebel fighters near the town of Hazano," Observatory director Rami Abdel Rahman told AFP.
He said six people from Hazano were also reported killed on Sunday, but it was unclear if they were civilians or fighters participating in the clashes.
The town lies in northwestern Idlib province, large parts of which lie under control of the Syrian opposition.
The Islamic State of Iraq and the Levant (ISIS), Al-Qaeda's Iraqi branch which has expanded into
Violence between the group and rebels affiliated with the mainstream Free Syrian Army broke out this week in the town of Azaz in northern Aleppo province close to the Turkish border.
Syrian rebel fighters initially welcomed the arrival of hardened jihadists affiliated with Al-Qaeda, but have turned against the hardline fighters in several places after abuses and disputes over tactics and ideology.
When ISIS announced it would expand into Syria, it initially said it planned to merge with an existing jihadist rebel force -- the Al-Nusra Front.
But Nusra, which has pledged allegiance to Al-Qaeda chief Ayman al-Zawahri, rejected the merger and there were reports that it had clashed with ISIS in northeastern Hasakeh province on Saturday.
