Israel says that it has hit dozens of targets in Syria, belonging to the government and allied Iranian forces.
The Israeli army says "large-scale strikes" have responded to missiles fired from an Iranian unit in Israel.
Syria claims that two civilians have been killed and that the Syrian air defense has dropped most missiles over Damascus. Other reports say the death toll has been higher.
Local reports say strong explosions have been heard in the capital. Photos on social media showed a number of fires.
On Tuesday morning, the Israeli army said it had intercepted four rockets fired from Syria in northern Israel. It says the rockets did not hit the ground.
Israel made hundreds of strikes in Syria after the outbreak of the civil war in 201
It is trying to thwart what it calls Iran's "military weapons" there and blocking the supply of Iranian weapons to the Lebanese Hezbollah movement.
But the latest operation was one of the most extensive attacks so far, says Barbara Plet Asher of the BBC in Jerusalem.
A senior Israeli security official said that the Israelis had taken a powerful revenge coup to signal that they were "changing the rules" – that even a small attack from Syria would elicit a broad response, our correspondent reported.
What did Israel say?
"During our strike on Iranian and Syrian terrorist targets, a Syrian anti-aircraft missile was launched, despite clear warnings to refrain from such fire. As a result, a number of Syrian air defense batteries were destroyed, "the IDF said.
The IDF also stated that it held" the Syrian regime responsible for actions taking place in Syria and warned them not to allow
Iran's Quds forces are the wing of the Islamic Revolutionary Guard Corps (IRGC) external operations.
Israel is not targeting advanced S-300 rocket systems located near positions. to Russian troops, Israeli media reported.
Russia, whose forces helped turn the tide of Syria's civil war in favor of President Bashar al-Assad, condemning Israeli strikes.
What did Syria say?
The Syrian state-run news agency Sanaa said "the country's air defense has opposed
It said that Syria had destroyed "most" of the Israeli missiles.
The news agency added that the Syrian territory had been hit by "Lebanese and Palestinian territories".
Israel has repeatedly struck targets in Syria by military aircraft in Lebanese airspace.
Danny Mackie, a British-Syrian journalist based in Damascus, posted footage of what he said he believed to be Israeli rockets hitting targets south of Damascus.
Syria has stated that two civilians were killed in Israeli strikes.
Meanwhile, a British watchdog group, the Syrian Observatory for Human Rights (SOHR), said 11 fighters, including seven foreigners, were killed.
SOHR says Israeli missiles have struck targets in and around Damascus in Kiswa, Sasaa, Meze Military Airport, Jdaidat Artouz, Kudsaya and Sahnaya.
What about Iran?
The Iranian authorities have not yet made any public comments.
What is the power of Quds?
The Force Quds (Jerusalem) responds directly to Iran's supreme leader, Ayatollah Ali Hamani.
It is headed by Major Mr. Kassem Solemimani, who is thought to be more than a military commander.
Following the US overthrow of Saddam Hussein's regime in Iraq in 2003, the Quds forces stepped up their operations in the Middle East, providing training, funding and weapons for
. He also developed forms of asymmetric war, such as swarming tactics, drones and cyberattacks that allowed Iran to undermine the superiority of its enemies in conventional weapons.
In April, US President Donald Trump designated the IRGC, including Quds Force, a "foreign terrorist organization" (FTO). For the first time, the US has designated a part of another government as the FTO.
Quds forces have about 5,000 personnel, according to a recently released report by the US military.