Israel's military on Monday said it has started hitting Iranian targets in Syria.
The Israel Defense Forces (IDF) says its operation is against the Quds Force - an elite unit of the Iranian Revolutionary Guards, the BBC reported.
It provided no details. But there are reports of strikes around the Syrian capital Damascus early on Monday.
Syrian media say air defences have repelled "an Israeli aerial attack". On Sunday, the IDF said it had intercepted a rocket over the Golan Heights.
The IDF announced the start of its operation in a tweet early on Monday.
The UK-based Syrian Observatory for Human Rights (SOHR) says Israeli rockets were targeting "the vicinity of the capital Damascus".
The SOHR adds that Syria's air defences were firing their missiles to intercept them.
Witnesses in Damascus spoke of hearing loud explosions in the night sky.
Israel rarely admits carrying out attacks on targets inside Syria.
But in May 2018, Israel said it had struck almost all of Iran's military infrastructure there in its such biggest assault since the start of the Syrian civil war in 2011.
Those strikes came after rockets were fired at Israeli military positions in the Golan Heights.