Jerusalem: Israel launched its most intensive air strikes yet against Iranian forces in Syria on Saturday after intercepting what it said was an Iranian drone and the rare downing of an Israeli F-16 warplane.

The incidents marked the most serious confrontations in Syria between Israel and Iranian-backed forces that have established a major foothold in the country while fighting in support of President Bashar al-Assad in a nearly seven-year-old civil war.

Iran's expanding clout during the Syrian civil war, including deployments of Iran-backed forces near the Israeli-occupied Golan Heights, has raised alarm in Israel, which has said it would act against any threat from its regional arch-enemy Tehran.

But both Israel and Syria signalled they were not seeking wider conflict, even as Prime Minister Benjamin Netanyahu rushed to military headquarters in Tel Aviv for consultations and the pro-Assad alliance pledged a strong response to any Israeli "terrorist action".

Russia's Foreign Ministry said on Saturday that Moscow, whose forces began intervening on behalf of Assad in 2015, was seriously concerned by the latest developments in Syria. It urged both sides to exercise restraint and avoid escalation.

"My impression is that it seems to be contained at this point," said a Western diplomat in the region. "I don't think anybody wants to escalate further."

The chain of events on Saturday began at 4.30am when an Israeli Apache helicopter shot down an Iranian drone over the northern town of Beit Shean, the Israeli military said.

The drone had been sighted taking off from a base in Syria, and was intercepted after it crossed into Israeli territory, said Lieutenant Colonel Jonathan Conricus, an Israeli military spokesman.

Israeli planes, including at least eight jets, then struck an Iranian installation in Syria which the Israeli military said operated the unmanned aircraft.

One of those planes, a US-made F-16, crashed on its return from that mission, landing in an empty field near Harduf, east of Haifa, in northern Israel.

Both pilots ejected over Israeli territory and were injured, one of them critically. The Israeli military said that early assessments showed it was hit by Syrian anti-aircraft fire, but could not immediately confirm that.

Air raid sirens sounded in the early morning of the Jewish sabbath in northern Israel and flights at Tel Aviv's international airport were briefly halted. The frontier fell quiet by mid-afternoon.

Iran rejected the Israeli version of events as "ridiculous". The pro-Assad military alliance denied that any of its drones entered Israeli air space.

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.

"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. Syrian air defences opened fire in response to an Israeli act of "aggression" against a military base, hitting "more than one plane", a military source said.

The military alliance fighting in support of Assad said Israel will witness a "severe and serious" response to its "terrorism" from now on, adding that Israel's claims a drone had entered Israeli airspace were a "lie".

The clashes marked a dangerous new confrontation between the international powers caught up in the conflict.

Iranian and Iran-backed Shiite 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.

Israeli Prime Minister Benjamin Netanyahu paid a rare visit to the Israel-Syria front this week and warned Israel's enemies not to "test" its resolve. He did not mention by name Iran or its Lebanese militia ally, both main players in Syria's civil war.

Netanyahu has been cautioning against any attempt by Iran to deepen its military foothold in Syria or construct missile factories in neighbouring Lebanon.

'Massive' anti-air fire

Israeli 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, said 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."

Flights temporarily suspended

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 Syrian Observatory for Human Rights said Israel had targeted areas in the countryside south-west of Damascus, near the Syrian-Lebanon border west of Damascus and in the eastern countryside of Homs province for several hours since dawn.

One set of raids hit positions belonging to the Syrian government and its allies in central Syria around the T4 airbase and in the Homs desert, the Britain-based Observatory said.

It said another set of raids hit south-west of Damascus, and another hit 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, though 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.

Civilian casualties spike

Syria and Russian airstrikes in rebel-held areas have killed 230 civilians in the past week in some of the conflict's worst violence that may also constitute war crimes, the top United Nations human rights official said on Saturday.

Zeid Ra'ad al-Hussein, UN High Commissioner for Human Rights, said his office had received reports including video footage of possible use of "toxic agents" on February 4 in the rebel-held town of Saraqeb, Idlib province.

"After seven years of paralysis in the (UN) Security Council, the situation in Syria is crying out to be referred to the International Criminal Court, as well as for a much more concerted effort by states to bring peace," Zeid said in a statement.

Reuters