Israel said it shot down a Syrian warplane that entered its airspace and that the jet crashed on the Syrian-held side of the Golan Heights.
The Israeli military said in a statement that two Patriot missiles were launched at a Syrian Sukhoi fighter jet, which it said had flown about 2 kilometres into skies above Israel, the BBC reported.
"It was shot down and it crashed," Israeli military spokesman Lieutenant-Colonel Jonathan Conricus told reporters. "We do not have any information so far about the pilots. I do not know of any reports of parachutes being spotted and we do not know if any pilots have been retrieved."
Syrian news agency Sana said Israel had targeted one of its jets over Syrian airspace, but did not say it was hit.
The agency quoted a military source as saying the plane was conducting raids against "armed terrorist groups" near the southern Yarmouk Valley.
Israel's Haaretz news website said residents in northern Israel saw interceptor missiles fired and heard explosions.
The Israeli military spokesperson said that the plane was Syrian, not Russian. "The only thing we know about the aircraft is that it crashed, most likely in the southern part of the Syrian Golan Heights," he said.
The Israeli Defence Forces said they were on "high alert" following "an increase in the internal fighting in Syria" and greater activity by Syria's Air Force in the region.
Israel captured the Golan Heights from Syria in 1967 and still occupies the area. The UN deployed a peacekeeping force between the two sides in 1974.
Syrian forces have been battling rebels and Islamic State militants at the frontier with Israel in recent weeks. Tuesday marked the first time government forces reached the border fence with the UN's Disengagement Observer Force at the edge of the Israeli-occupied Golan Heights.
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