ISRAEL has shot down an infiltrating Iranian drone and struck Iranian targets deep in Syria before one of its own jets was shot down.
The engagement is Israel's most serious yet in neighbouring Syria since fighting there began in 2011.
Israel called the drone infiltration a "severe and irregular violation of Israeli sovereignty" and warned that Iran would be held accountable for its meddling.
This raises the spectre of a larger confrontation in an area that has remained largely stable since a month-long war between Israel and Hezbollah in Lebanon in 2006.
Israel would not confirm whether its aircraft was shot down by enemy fire, which would mark the first such instance for Israel since the first Lebanon war in 1982.
The sudden escalation offers what could be a harbinger of events as the Syrian fighting winds down and an emboldened Iran establishes a military presence that Israel vows it will never accept.
"This is a serious Iranian attack on Israeli territory. Iran is dragging the region into an adventure in which it doesn't know how it will end," said Israel's chief military spokesman, Brig Gen Ronen Manelis.
"Whoever is responsible for this incident is the one who will pay the price."
Prime Minister Benjamin Netanyahu and defence minister Avigdor Lieberman convened the top brass at military headquarters in Tel Aviv for long hours of emergency consultations throughout the Jewish Sabbath to discuss their next steps.
Mr Netanyahu said he spoke with Russian president Vladimir Putin and vowed to strike back hard.
Israel said it shot down an unmanned Iranian aircraft that penetrated its airspace and then destroyed the Iranian site in central Syria that it said launched it.
On their return, Israel's jets came under heavy Syrian anti-aircraft fire and the pilots of one of the F-16s had to eject and the plane crashed in northern Israel. One pilot was seriously wounded and the other one lightly.
In subsequent attacks, the Israeli military said it struck four additional Iranian positions and eight Syrian sites, causing significant damage.
The Israeli jets again faced a heavy barrage of anti-aircraft missiles but returned home safely, as large explosions were reported in Syria and warning sirens blared in northern Israel.
Israel says the strikes destroyed the main command and control bunker of the Syrian military and marked its most devastating assault against Syria in decades.
Iran denied Israel's shooting down of a drone, with foreign ministry spokesman Bahram Ghasem calling the account "ridiculous," while the joint operations room for the Syrian military and its allies insisted the drone had not violated Israeli airspace and was on a regular mission gathering intelligence on Islamic State militants.
"Iran seeks to use Syrian territory to attack Israel for its professed goal of destroying Israel," said Mr Netanyahu.
"Israel holds Iran and its Syrian host responsible for today's aggression. We will continue to do whatever is necessary to protect our sovereignty and our security."
Israel also appealed to the United Nations Security Council to denounce Iran's aggression and "put an immediate end to Iranian provocations."
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 Hezbollah - Lebanon's Iranian-backed political party and 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 - which is fighting alongside Syrian forces - almost 100 times since 2012.
But Israel has refrained from striking Iranian sites directly. Syria has also repeatedly said it will respond to Israeli airstrikes but has rarely returned fire. Both of those trends came to an abrupt end on Saturday as a rapid escalation played out.
Hezbollah said Saturday's developments signalled a "new strategic phase" in engaging Israel, which has been mostly off the guerrilla group's radar as it has been heavily engaged in the fighting in Syria.
Former Syrian lawmaker and political commentator Sharif Shehade said the anti-aircraft fire marked a political decision in Syria to respond to Israeli strikes, and that tensions would continue to rise unless Washington and Moscow intervened to calm the situation.
"It is a decisive decision to confront the Israeli air force and its careless behaviour," he said. "I think what happened today is a lesson for Israel."
Military spokesman Lt Col Jonathan Conricus said Saturday's incident marked the most "blatant and severe violation of Israeli sovereignty" yet. He said while Israel had no interest in further escalation it was prepared to "extract a heavy price" for such aggression.
By directly sending the drone into Israel on a military mission, Lt Col Conricus said Iran was "playing with fire."
Russia, which backs Assad and maintains a large military presence in the country, called for restraint and appeared to criticise Israel's actions.
"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 to assist in the fight against terrorists," Russia's foreign ministry said.
Israel has shot down several drones that previously tried to infiltrate its territory from Syria.
The capture of an Iranian drone and the targeting of Iranian sites in response, however, marked a dramatic escalation in the Israeli retaliation.
The military confirmed that the initial target in Syria - the unmanned aircraft's launch components - was successfully destroyed and released grainy footage of the strike. Israel also said it had recovered the remnants of the drone, which proved that it belonged to Iran.
