The Iranian commander, identified as Farhad Dabirian, was killed by a silenced gun in the Sayyidah Zaynab area south of Damascus, said the Syrian Observatory for Human Rights.

The Britain-based watchdog said Dabirian had led operations on the border with the Israeli-occupied Golan Heights, and was in charge of the military operation in Palmyra against the Islamic State (IS).

He was also close to Hassan Nasrallah, secretary-general of the Lebanese Shiite Hezbollah group.

The observatory gave no further details about the assassin, but several Israeli airstrikes on Syria targeted pro-Iranian fighters in Syria.

Iran has been a key backer of the Syrian government forces in the nine-year-long war, which raised the ire of Israel whose leaders have repeatedly vowed no tolerance of any Iranian influence in Syria.