The Israeli Air Force downed a Syrian MiG-21 fighter jet over the Golan Heights as it began to infiltrate Israeli territory, according to military sources.

"It was a Russian-made Sukhoi," said Israeli military spokesman Peter Lerner. "The circumstances of the incident are being reviewed."

"This is developing, we'll have to see where it goes," he said.

The downing of the Syrian jet is the first such incident for three decades and Israeli sources claimed that the jet was not on its way to attack Israeli targets but had passed into Israeli airspace by accident.

Israel Radio claimed that the aircraft was battling the moderate Syrian rebels who are attempting to topple Syrian President Bashar al-Assad.

Unconfirmed photographs of the aircraft that was shot down were posted by social media users.

Fighting in the 4-year Syrian civil war has spilled over into the Golan Heights with Israel shooting down a Syrian drone earlier this year and previously having to scramble jets to the Golan Heights without having to open fire.

Rockets have landed on the Israeli side of the border between the two countries as the Syrian regime forces and moderate rebels battle for the Quneitra crossing.