A senior Iranian official says Israel's airstrike on a Syrian air base that killed seven Iranians is "Israel's crime" and "will not go unanswered."

Ali Akbar Velayati, an aide to Iran's supreme leader, made his comments upon arrival in the Syrian capital Damascus on Tuesday.

Iran is one of Bashar Assad's strongest backers and has sent thousands of Iran-backed gunmen to back his forces in an advisory role, but hundreds have been killed so far.

Velayati was quoted by the country's state-run news agency IRNA.

Russia, Iran and Syria have blamed Israel for Monday's attack on the T4 air base in the central province of Homs that a Syria war monitor said killed 14 people, including Iranians.

Iran's semi-official Tasnim news agency puts the number of Iranians killed in a missile attack on a Syrian air base at seven. The Tuesday report said bodies of victims have been transferred to Tehran. It said funerals will be held in the victims' cities of origin across the country. Two of them, Hamed Rezai and Hojjatollah Nochamani, are from Tehran. The report did not provide the victims' military rank or further information.

On Monday, Iran identified four of the killed, including a Revolutionary Guard colonel.