The top adviser to Iran's Supreme Leader Ali Khamenei on Tuesday described a strike on a military air base in Syria as "Israel's crime" and said it would "not remain without response", the Lebanese al-Mayadeen channel reported.
Ali Akbar Velayati was speaking on arrival in the Syrian capital Damascus, reported al-Mayadeen, which is well-connected in government-held parts of Syria.
Seven Iranian military personnel were killed in the strike on the Tiyas, or T-4, air base near Homs, the Iranian Tasnim news agency reported on Tuesday.
Syria, Iran and Russia have accused Israel of being behind the attack - something Israel has neither confirmed nor denied.
However, Israel has carried out numerous strikes in Syria, with some intended to stop Iran and allies including the Lebanese group Hezbollah from gaining strength near its borders.
Iran's military help has been crucial for Syrian President Bashar al-Assad in his war against rebel groups since 2011, and Velayati also visited Syria in November.
Israel regards Iran as its biggest external foe and has called Hezbollah the biggest threat on its borders.
The enmity between Israel and Iran has threatened to escalate as a result of their competition in Syria. In February, Syrian anti-aircraft fire brought down an Israeli F-16 jet that was attacking the positions of Iran-backed forces in Syria.
On Monday, the UN Syria envoy Staffan de Mistura referenced the air base strike along with other recent events in Syria in a briefing to the Security Council, warning against a "situation of uncontrollable escalation".