Israel signalled that the plane's infiltration may have been the result of internal fighting in Syria's civil war, but stressed it would enforce the ceasefire lines between the two countries.

"This is a blatant violation of the 1974 separation agreements between Israel and Syria," Israeli Prime Minister Benjamin Netanyahu said in a statement.

"We will not accept any intrusion or spillage into our territory, either from the ground or from the air."

The plane, either a Sukhoi 22 or 24, crashed on the Syrian side of the fence in the Golan Heights and it was unclear what happened to the pilot or pilots, Israeli military spokesman Jonathan Conricus told journalists.

It was hit after it penetrated some two kilometres (1.25 miles) into Israeli-controlled airspace in the southern Golan Heights, said Conricus.

A Syrian military source confirmed Israel had fired at one of its warplanes but said the fighter had been carrying out operations against jihadists over Syrian territory.

Israel "targeted one of our warplanes... in Syrian airspace", the source said, cited by state news agency SANA.

State media in Damascus gave no account of the fate of the plane or the pilots.

According to Conricus, the plane left from the T4 airbase in central Syria before heading towards the Israeli-occupied Golan.

Israel's military monitored the plane throughout and shot it down with two Patriot missiles, Conricus said.

It was the first time Israel shot down a manned Syrian fighter jet since 2014.

- 'High alert' -

"From our perspective, the event is behind us," Conricus said. "However we of course remain on high alert."

Israel's army said there had been an increase in "internal fighting in Syria" from early Tuesday.

"We issued warnings and messages through different channels in various languages, numerous times over the day, in order to avoid any miscalculations, misunderstandings or any violations of Israeli airspace," Conricus said.

The Syrian Observatory for Human Rights said that bombing operations in the border area by both Syrian and Russian aircraft had dramatically decreased after the incident.

The Britain-based war monitor said there had been heavy bombing earlier on the Yarmuk river basin in the south but that had all but stopped by the evening, dropping to just one strike per hour.

The Syrian military source said the targeting of the warplane was proof of Damascus's repeated accusation that Israel supports "terrorists" in Syria.

"The Israeli enemy has confirmed its adoption of armed terrorist groups, and targeted one of our warplanes which was hitting them," the source said.

Israel has been stressing for weeks that it would enforce the ceasefire line between it and Syria amid a Russian-backed government offensive in the country's south.

Tuesday's incident comes a day after Israel's air defences fired at Syrian rockets it feared could hit its territory.

Netanyahu also met Russian Foreign Minister Sergei Lavrov in Jerusalem on Monday to discuss the Syrian conflict.

Israel has been on high alert since June 19, when Syrian government forces launched the offensive to retake Quneitra and Daraa provinces, adjacent respectively to the Israeli-held section of the Golan and to Jordan.

It has sought to avoid direct involvement in Syria's seven-year civil war, but it has acknowledged carrying out dozens of air strikes there to stop what it says are advanced weapons deliveries to the Shiite Lebanese militia Hezbollah, one of its top enemies.

- 'All forces must leave' -

It has also pledged to prevent its arch-enemy Iran from entrenching itself militarily in the neighbouring country.

A series of air strikes that have killed Iranians in Syria have been attributed to Israel in recent weeks.

The strikes have led to condemnation from Syrian President Bashar al-Assad's regime, but Israel has maintained good relations with Russia and has coordinated its actions in Syria with Moscow.

Both Russia and Iran are backing Assad in the civil war.

Conricus declined to say whether Israel informed Russia before shooting down the fighter jet, but said there had been conversations on a hotline between the two countries to avoid accidental clashes in Syria throughout the day.

In Monday's meeting between Lavrov and Netanyahu, Russia offered to keep Iranian forces 100 kilometres (62 miles) away from the Israeli-occupied Golan, but Israel said the proposal did not go far enough.

"We won't accept Iranian military entrenchment in Syria, not near the border, not beyond the 100-kilometre stretch, which by the way the Russians talk about and agree to," a senior Israeli official said, speaking on condition of anonymity.

"We said there are also long-range weapons beyond that distance, and all the forces must leave Syria."

Israel seized 1,200 square kilometres (460 square miles) of the Golan Heights from Syria in the 1967 Six-Day War, in a move never recognised internationally.

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