Israel's intensive air raids against what it said were Iranian targets inside Syria have ratcheted up regional tensions, as Russia, a key player in Syria's long-running war, urged restraint.

The Israeli army on Saturday launched "large-scale" attacks inside Syria after intercepting what it said was an Iranian drone and the subsequent downing of an Israeli fighter jet, which crashed after it came under fire from Syrian air defence forces.

The incidents marked the most serious confrontation yet between Israel and Iran since the civil war in Syria began in 2011.

In Russia, whose forces began intervening in support of Syria's President Bashar al-Assad in 2015, officials said they were "seriously concerned" by the confrontation.

"We urge all parties involved to exercise restraint and to avoid any actions that could lead to an even greater complication of the situation," Russia's foreign ministry said in a statement.

"We consider it necessary to unconditionally respect the sovereignty and territorial integrity of Syria and other countries of the region," it added.

The Israeli air raids began early on Saturday when the country's military said it had intercepted an unmanned aircraft over the city of Beit Shean "on a military mission sent and operated by Iranian military forces".

It retaliated by sending at least eight jets to attack Syrian and Iranian military bases, which according to the Syrian Observatory for Human Rights were located near Homs province.

One of those planes, an F-16 fighter jet, crashed on return from that mission. The two pilots from the jet were injured, one of them critically.

Israel responded with new attacks on bases in southern Syria, hitting at least 12 positions in Syria, including three aerial defence batteries.

Benjamin Netanyahu, Israel's prime minister, said he spoke to Russian President Vladimir Putin over the phone on Saturday evening and "repeated to him our right and duty to defend ourselves".

In a televised statement, Netanyahu said Israel "seeks peace but we will continue to defend ourselves steadfastly against any attack against us or any attempt by Iran to establish itself against us in Syria".

Red lines

Israel has issued several warnings about the presence of Iranian forces in Syria. It fears Iran could use Syrian territory to stage attacks or transfer weapons to the Lebanese group, Hezbollah.

Israel has struck weapons convoys destined for Hezbollah, which is fighting alongside Syrian forces, almost 100 times since 2012.

The Syrian military and its allies denied that the drone violated Israel's airspace, Syria's state-run SANA news agency reported.

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The joint operations room, run by Syria, Iran, Russia and Hezbollah, said the aircraft was on a regular mission gathering intelligence on fighters of the Islamic State of Iraq and the Levant (ISIL).

The statement also warned Israel that it will respond "crushingly and seriously".

Al Jazeera's Zeina Khodr, reporting from Beirut in Lebanon, said Saturday's confrontation showed Syria and its allies were no longer willing to tolerate Israeli attacks inside the country.

"Israel has repeatedly said it won't allow Iran and its allies to entrench themselves militarily in Syria," she said.

"Netanyahu calls them red lines. Now the Syrian government and its allies have introduced red lines of their own. They said the shooting down of the Israeli jet is a clear message that the days of carrying out attacks inside Syria are gone."

Meanwhile, Hezbollah, a Shia group which is represented in the Lebanese parliament and has a strong armed wing, hailed Syria's response to Israel's attack on Iranian and Syrian bases in Syria, saying it signals "a new strategic phase" that puts an end to the violation of Syrian territories.

In a statement published by Lebanon's NNA news agency, Hezbollah said the developments denote "the categorical fall of ancient equations".

Lebanon protested Israel's use of its airspace to target Syria, saying it will complain to the United Nations Security Council.

Its foreign ministry also said in a statement that it supported Syria's "legitimate" right to respond to Israel's "aggressive policy", which threatened stability in the region.

Separately, Iran also denounced Israeli "lies" and said Syria had the right to self-defence in response to Israeli air raids.

"Shooting down the Israeli jet is a clear message that the days of carrying out attacks inside Syria are gone. Any new attack will be confronted. Iran stands by Syria against any foolish move by Israel," the country's foreign ministry said.

In Washington, the US said it "fully supports" Israel's right to defend itself against threats, while in Gaza in the occupied Palestinian territories, the armed wing of Hamas declared full alert among its fighters because of the escalation between Syria and Israel.