Israel shot down a Syrian fighter jet that penetrated Israeli airspace on Tuesday, the Israeli army said.
It said that the Russian-made Sukhoi plane was under surveillance when it flew some 2 kilometers (1.25 miles) into Israeli airspace.
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Syria said in response that one of its jets was targeted by Israel while conducting raids in southern Syria, in Syrian airspace.
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Israel's air defenses fired two Patriot missiles at the jet. IDF Spokesman Brig. Gen. Ronen Manelis said the plane was either a Sukhoi-22 or -24 and that it crashed in Syrian territory in the southern portion of the Syrian Golan Heights, although the fate of the pilots was unknown. A non-Syrian source close to the Assad government, however, said the pilot was killed.
The Israeli military said the Syrian plane took off from the T-4 airbase near Homs, which was the target of at least two Israeli airstrikes in the past. The base was used by Iranian forces and housed a command center for Iran's drone operations in Syria.
Prime Minister Benjamin Netanyahu echoed the military's statement, calling it "a gross violation" of the existing agreement between the two countries.
"Our air defense systems identified a Syrian Air Force jet that took off from the T-4 Syrian Air Force base and penetrated Israeli airspace," Netanyahu said. "This is a gross violation of the 1974 Separation of Forces Agreement with Syria. I have reiterated and made clear that we will not accept any such violation. We will not accept any such penetration of, or spillover into, our territory, neither on the ground nor in the air. Our forces acted appropriately. We insist that the Syrians strictly abide by the Separation of Forces Agreement between us and them."
In Tuesday's incident, sirens sounded in Israeli communities across the Golan Heights, near the Syria border. Residents in northern Israel reported seeing interceptor missiles being fired from the Safed area and reported hearing explosions.
The Israeli army said in a statement that Tuesday morning saw internal fighting in Syria intensifying, "including increased activity by Syrian air forces. The Israel Defense Forces is ready and vigilant, and will continue to act against violations of the 1974 Separation of Forces Agreement.
"Over the years of the fighting in Syria, the Israeli army has not been involved in the fighting. But in recent days the fighting has gotten closer to Israeli territory," the IDF spokesman added. "Today too, we are sending messages through a number of channels in a number of languages that we will not agree to accept a violation of the 1974 Separation of Forces Agreement."
He said that from Israel's standpoint, Syria is responsible for what happens in its territory.
During the course of the day, including during the incident, Israel was in touch with Russian officials involved in coordination with Israel, Manelis noted.
This is the second time the Israel Air Force has brought down a Russian-made Syrian jet since 1985.
The first time was in 2014, when it shot down a Sukhoi-24 that entered Israeli airspace over the Golan Heights. The same defense battery was involved in that incident as Tuesday's.
In 2014, the plane was intercepted by a Patriot missile some 800 meters west of the Israel-Syria border. The decision to intercept that plane was taken within 80 seconds, and by the time the interceptor missile hit the jet, it was already on its way back to Syrian airspace. The two pilots ejected and landed in Syrian territory.
This Monday, Israel used its David's Sling missile-defense system operationally for the first time, firing at two Syrian surface-to-surface missiles. An initial army investigation found that the system identified the two Russian-made SS-21 missiles, calculating that they were likely to land south of Lake Kinneret. A senior Israeli officer made the decision to use the system.
In that incident, one of the interceptors was ordered to self-destruct after it became clear that its target was expected to fall in Syrian territory. It has since been cleared for publication that the second interceptor fell in Syrian territory after missing its target.
If it is decided that an interceptor will not hit its target after launch, the Israeli military's preference is to destroy the interceptor at high altitude – in order to avoid it falling into enemy territory, where information can be gleaned from it.
The fact that the second interceptor fell in Syrian territory without hitting its target or being destroyed by Israeli forces is a mishap that the Israeli army is now investigating. It is unclear if it was the result of decision-making or a problem with the interceptor itself.
In a related development, Russia is working to ensure the removal of Iranian forces to 100 kilometers (62 miles) from the Golan Heights. However, Israel is demanding that long-range weapons that could circumvent such a buffer zone also be withdrawn, a senior Israeli official said Monday after a meeting between Netanyahu and visiting Russian Foreign Minister Sergey Lavrov and Russian military Chief of Staff Gen. Valery Vasilyevich Gerasimov.
According to the GlobalFirepower website, the Syrian army has 235 fighter jets and 167 helicopters, including 28 attack helicopters, as well as 154,000 troops in active service. Syria's annual defense budget is said to be nearly $1.9 billion.