The Russian-made Sukhoi fighter was shot down by a Patriot missile, military spokeswoman Lieutenant Libby Weiss said. The jet was identified as it crossed over the Golan frontier. A missile was launched as it was turning around and the warplane crashed inside Syria, an Israeli official said, speaking anonymously because he was not authorized to comment on record.

“We have clarified in the past, and we emphasize this even now: We will not allow any party whatsoever, whether it be a nation or terrorist organization, to threaten our security and violate our sovereignty,” Israeli Defense Minister Moshe Ya’alon said in an e-mailed statement.

The downing of the jet is the most serious incident between the sides since the outbreak of the Syrian civil war more than three years ago. Mortars and shells fired by forces fighting in Syria have landed in the Israeli-held Golan several times over the past year, often prompting Israeli retaliatory fire across the border at Syrian military positions.

This the first Israeli downing of a Syrian fighter jet since 1985 when aircraft from the two countries engaged in a dogfight over Lebanon.

“While this incident reflects the growing tensions on the Syrian border since the start of their civil war, I don’t think it will necessarily lead to any kind of escalation,” said brigadier-general Shlomo Brom, a senior research fellow at the Institute for National Security Studies at Tel Aviv University. “We’ve seen several similar incidents involving Syria and Turkey, and when you have a conflict going on next to you, these mistakes happen.”

The Golan incident comes just as the U.S. launched its first air strikes against Islamic State positions in Syria.