Israel says it has hit Iranian targets around the Syrian capital, Damascus.
The Israel Defense Forces says the overnight operation targeted the elite Quds Force of the Iranian Revolutionary Guards, as well as Syrian air defences.
The Syrian military says it shot down most of the Israeli missiles. But a monitoring group reported that at least 11 pro-government fighters were killed.
The IDF says it acted after the Quds Force fired a rocket from Syria towards the occupied Golan Heights on Sunday.
Iran, Israel's arch-enemy, has backed Syria's President Bashar al-Assad in the country's civil war and has sent thousands of military personnel to support him.
What do we know about the Israeli operation?
The IDF rarely admits carrying out attacks inside Syria. But it announced the start of Monday's strikes on the Quds Force in a tweet.
A statement issued later said Israeli fighter jets had hit munition storage sites, a site located in Damascus International Airport, an Iranian intelligence site and an Iranian military training camp.
The jets also struck several Syrian military air defence batteries after dozens of surface-to-air missiles were launched "despite clear warnings", according to the IDF.
"In firing towards Israeli territory yesterday [Sunday], Iran once again offered definitive proof of its real intentions of its entrenchment in Syria, which endangers the State of Israel and regional stability," the statement said.
"The IDF will continue operating determinedly to thwart Iranian entrenchment in Syria.
"In addition, the IDF holds the Syrian regime responsible for everything taking place within Syria and warns the Syrian regime against targeting Israel or permitting it to be targeted."
What does the Syrian military say?
Syria's state-run Sana news agency quoted a military source as saying that the country's air defences "confronted an Israeli attack" and "prevented it from achieving its objectives".
"Our air defences dealt with the situation and intercepted the hostile missiles, downing most of them before reaching their targets as they continue their heroic response to the aggression," the source added, without mentioning any casualties.
The Russian military, which also supports the Syrian government, said more than 30 missiles and guided bombs were shot down by Syrian air defences.
However, it added that the "infrastructure" of Damascus International Airport was partially damaged and that four Syrian soldiers were killed and six were wounded.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said the strikes had targeted weapons depots belonging to Iran and the Iran-backed Lebanese militant group, Hezbollah.
At least 11 fighters, including two Syrians, were killed, it added./BBC