Israeli Prime Minister Benjamin Netanyahu (C) sits next to Cabinet Secretary Tzachi Braverman (R) and Israeli Intelligence and Transportation Minister Israel Katz (L) during a cabinet meeting at the Prime Minister's office in Jerusalem on February 11, 2018.

Israel's Minister of Intelligence Israel Katz said on Sunday that by striking key Iranian sites in Syria, Israel sent a clear message to Iran that it would not tolerate an Iranian military foothold on its doorstep and would act decisively to counter any further provocations.

The wave of airstrikes came after Israel intercepted an Iranian drone that had infiltrated its airspace, and an Israeli F-16 was downed upon its return from Syria on Saturday. It was Israel's most serious engagement in neighboring Syria since fighting there began in 2011 — and the most devastating air assault on the country in decades.

The military said it destroyed the drone's Iranian launching site, along with four additional Iranian positions and eight Syrian sites, including the Syrian military's main command and control bunker.

The Syrian Observatory for Human Rights, which monitors the war in Syria through a network of activists on the ground, said Sunday that at least six Syrian troops and allied militiamen were killed in the airstrikes. The six included Syrian troops as well as Syrian and non-Syrian allied troops, the Britain-based Observatory said.

"They, and we, know what we hit and it will take them some time for them to digest, understand and ask how Israel knew how to hit those sites," Katz told Israel's Army Radio. "These were concealed sites and we have intelligence agencies and the ability to know everything that is going on there and yesterday we proved that."

In Saturday's attacks, the Israeli jets came under heavy Syrian anti-aircraft fire and the pilots of one of the F-16s had to eject before the plane crashed in northern Israel. One pilot was seriously wounded and the other one lightly.

Israel would not confirm whether its aircraft was actually shot down by enemy fire, which would mark the first such instance for Israel since 1982 during the first Lebanon war.

Israel has recently issued several stern warnings about the increased Iranian involvement along its borders with Syria and Lebanon, which it attributes to Iran's growing confidence following Syrian President Bashar Assad's successes in the Syrian civil war, thanks to support by main allies Russia and Iran.

Israel fears Iran could use Syrian territory to stage attacks or create a land corridor from Iran to Lebanon that could allow it to transfer weapons more easily to the Lebanese Hezbollah — an Iranian-backed Shiite militant group sworn to Israel's destruction.

Though Israel has largely stayed out of the Syrian conflict, it has struck weapons convoys destined for Hezbollah dozens of times since 2012.

Israel has also shot down several drones that previously tried to infiltrate its territory from Syria, but the capture of an Iranian drone and the direct targeting of Iranian sites in response marked a dramatic escalation in the Israeli retaliation.

Former Defense Minister Moshe Yaalon told Army Radio that Iran was caught off guard by Israel's actions.

"The Iranians thought that the drone they were flying toward us had a low signature, meaning that radar wouldn't detect it," said Yaalon. "They were really surprised."

Israel has long complained about the involvement of archenemy Iran, and Iranian proxy Hezbollah, in the Syria war. Both have sent forces to back Assad, who appears headed toward victory after years of fighting. Israel has said it will not accept a permanent military presence by Iran and its Shiite allies in Syria, especially near the Israeli border.

Prime Minister Benjamin Netanyahu has held several consultations with Russian President Vladimir Putin, who backs Assad's government and maintains a large military presence in Syria. Following the Israeli strikes they spoke again on Saturday, with Netanyahu conveying Israel's determination to counter Iran's intentions.

Still, Russia's foreign ministry appeared to criticize Israel's actions by calling for restraint and respecting Syria's sovereignty.

"It is absolutely unacceptable to create threats to the lives and security of Russian servicemen who are in Syria at the invitation of its legitimate government," it said.

The United States, on the other hand, strongly backed Israel.

"Iran's calculated escalation of threat and its ambition to project its power and dominance places all the people of the? region — from Yemen to Lebanon — at risk," said Heather Nauert, the State Department spokeswoman.? "The U.S. continues to push back on the totality of Iran's malign activities in the region and calls for an end to Iranian behavior that threatens peace and stability."