Russia has said Syria shot down one of its military planes - but laid the blame for the deaths of the 15 personnel on board with Israel.
The defence ministry said Israeli jets put the Il-20 plane into the path of Syrian air defence systems on Monday after failing to give Moscow enough warning of a strike on Syrian targets.
The Il-20 disappeared off the radar at about 23:00 local time (20:00 GMT).
The Israel Defence Force (IDF) has expressed "sorrow" over the deaths.
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However, in a statement released on Twitter, it added: "Israel holds the [Syrian President Bashar al-]Assad regime, whose military shot down the Russian plane, fully responsible for this incident."
It went on to say its jets were back in Israeli airspace by the time the missiles were launched.
Israel - which also blamed Iran and Hezbollah - rarely acknowledges carrying out strikes on Syria, but an Israeli military official recently said it had hit more than 200 Iranian targets in Syria over the past 18 months.
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The Israeli government is concerned by what it calls Iran's "military entrenchment" in Syria, as well as shipments of Iranian weapons to the Lebanese militant group Hezbollah, which is fighting alongside Syrian government forces.
What exactly happened?
The details are murky, and Russia's account of the incident has yet to be verified.
Monday's incident is reported to have occurred about 35km (22 miles) from the Syrian coast as the Ilyushin Il-20 aircraft was returning to Russia's Hmeimim airbase near the north-western city of Latakia.
Russia's Tass news agency says the Il-20 plane "disappeared during an attack by four Israeli F-16 jets on Syrian facilities in Latakia province".
Reports on Syrian state media spoke of an attack in the area shortly before the plane disappeared. According to Sana news agency, the military said it had intercepted "enemy missiles coming from the open sea towards the city of Latakia".
Syrian television also reported explosions over the sky in Latakia just before 22:00. Thirty minutes later, the Sana Facebook page reported that Syrian air defences had responded to enemy missiles.
What does Russia accuse Israel of doing?
In a statement, Russia said Israel's "irresponsible actions" were to blame, saying it was given less than a minute's warning ahead of the strikes, which was not enough time to get the military surveillance plane out of the way.
"The Israeli planes deliberately created a dangerous situation for surface ships and aircraft in the area," a defence ministry spokesman said.
The spokesman accused Israeli pilots of "using the Russian airplane as a cover", putting it "in the line of fire coming from Syrian air defence systems".
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The plane, the spokesman continued, was shot down by a Syrian missile.
"As a result of the irresponsible actions by the Israeli military, 15 Russian servicemen have died," the spokesman said.
It is not possible to verify any of these claims.
What has Israel said?
The IDF said its jets were targeting Syrian military facilities "from which systems to manufacture accurate and lethal weapons were about to be transferred on behalf of Iran to Hezbollah in Lebanon".
It added the Syrian anti-aircraft missiles were "inaccurate" and "extensive". What's more, it claimed its jets were back in Israeli airspace by the time the missiles were launched.
The statement also accused the Syrian military of failing "to ensure that no Russian planes were in the air", adding it would share the relevant information with Moscow.
In an earlier phone call on Tuesday, Russian Defence Minister Sergei Shoigu told his Israeli counterpart Avigdor Lieberman the blame "fully rests with Israel", adding that Russia "reserves the right to take further steps in response".
Russia later summoned the Israeli ambassador to its foreign ministry.
A search-and-rescue operation is under way.