JERUSALEM/BEIRUT - Israel launched heavy air strikes in Syria on Saturday, saying it hit air defenses and Iranian targets, and the Syrian army claimed to have brought down an Israeli F-16 that crashed in northern Israel in a major escalation of tension.The Israeli military said early assessments indicated the jet had been shot down by Syrian fire, but this was still unconfirmed.It marked the most serious confrontation yet in Syria between Israel and Iranian and Iran-backed forces that have established a major foothold in the country while fighting in support of President Bashar Assad in the civil war.Israel said the F-16 crashed during a mission to strike Iranian drone installations in Syria. It said it sent its jets into Syria after shooting down an Iranian drone over Israeli territory earlier on Saturday.The military alliance fighting in support of Assad denied any of its drones had entered Israeli air space. In a statement, it said Israel had targeted an air base in the Homs desert that is being used to fly drones in missions against Islamic State.Such "terrorist action" by Israel would be met with a "severe and serious response," it said.The Israeli military spokesman said Israel did not seek escalation in the region, calling its action a "defensive effort triggered by an Iranian act of aggression."Iran's expanding clout during Syria's nearly seven-year-long war, including deployments of Iran-backed forces near the Golan frontier, has raised alarm in Israel, which has said it would act against any threat from its regional arch-enemy Tehran.Iranian and Iran-backed Shi'ite forces, including Lebanon's Hezbollah, have deployed widely in support of Assad. Iran's military chief warned Israel last October against breaching Syrian airspace and territory.Israel's air force has targeted Syrian military and Hezbollah targets in Syria on an almost regular basis, but its attacks on Saturday appeared to be the most intense yet.Referring to the downed Israeli F-16, an official in the pro-Assad alliance said a "message" had been delivered to Israel. "I do not believe matters will develop to a regional war," the official said. US HAWKISH ON IRANIn Washington, President Donald Trump’s administration has backed Israel’s hawkish stance on Iran, and declared containing Tehran's influence an objective of its Syria policy. On a visit to Israel last month, US Vice President Mike Pence called Iran the world’s "leading state sponsor of terror."US Secretary of State Rex Tillerson is also expected to visit the region in the coming week to discuss the crisis in Syria and other issues, and is scheduled to visit Jordan, Turkey, Lebanon and other countries.Hezbollah and Israel last fought a major conflict in 2006. Tensions have also spiked across the frontier between Israel and Lebanon over Israeli plans for border wall, and Lebanese plans to exploit an offshore energy block which is partly located in disputed waters.The Israeli military said 12 targets, including three aerial defense batteries and four Iranian targets that are part of Iran's military establishment in Syria were attacked."During the attack, anti-aircraft missiles were fired towards Israel, triggering alarms that were heard in Northern Israel," the military said.Syrian state media reported two separate Israeli attacks.In the first one, a military source said Syrian air defenses had opened fire in response to an Israeli act of "aggression" against a military base, hitting "more than one plane."Later, state media said air defenses were responding to a new Israeli assault and air defenses had thwarted attacks on military positions in southern Syria.Israel said one of its attack helicopters shot down an Iranian drone at around 4.30am (0230 GMT) that had come from Syria into Israel. "In response, the IDF (Israel Defense Forces) targeted Iranian targets in Syria," the military said. "MASSIVE" ANTI-AIRCRAFT FIREIsraeli military spokesman Jonathan Conricus said a "substantial" number of Israeli warplanes on the mission had come under "massive Syrian anti-air fire," and only one Israeli jet was harmed.The F-16 came down in a field near the northern Israeli village of Harduf, television footage showed, and one of the pilots was injured as they ejected, the military said.David Ivry, a former Israeli Air Force chief, told Reuters he believed it was the first time an Israeli F-16 was brought down since Israel began using the jets in the 1980s."We don't know if the pilots ejected because of the (Syrian)fire," Conricus said. It was also unclear at what stage of the mission they ejected, he said, "but it is of extreme concern to us if they were shot down."The Syrian Observatory for Human Rights said Israel had targeted areas in the countryside southwest of Damascus, near the Syrian-Lebanon border west of Damascus and in the eastern countryside of Homs province for several hours since dawn.It said another set of raids hit southwest of Damascus, and another struck around the Damascus-Beirut highway near the border with Lebanon.In Israel, uniformed military personnel could be seen gathered around the burnt and tangled metal in Harduf by mid-morning, with what appeared to be white foam on the surrounding grass. Others knelt in the grass, inspecting pieces of the jet.Rocket alert sirens sounded in the Israeli-held Golan Heights and in northern Israel. There were no reports of casualties.Flights in to Israel's main airport near Tel Aviv were suspended for about 15 minutes and take-offs were held for about 20 minutes on Saturday morning. "Ben Gurion Airport is now operating as usual," Israeli Airport Authority spokesman Ofer Lefler said.The airport's online live flight schedule showed flights were departing and landing.