Russia has said Syria shot down one its military planes - but laid the blame for the deaths of the 15 personnel on board with Israel.
The defence ministry said Israeli jets forced 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).
Israel is yet to comment on the allegations.
Israel 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.
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.
The incident on Monday reportedly 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.
According to Russia's Tass news agency, the Il-20 plane "disappeared during an attack by four Israeli F-16 jets on Syrian facilities in Latakia province".
On Tuesday, the Russian defence ministry said the aircraft was shot down by Syria.
But it said Israel's "irresponsible actions" were to blame, saying it was given just a minute's warning ahead of the strikes, which was not enough time to get the military surveillance plane out of the way.
Exactly what happened to the aircraft is not yet known. It was last seen on the radar as it was returning to its base near Latakia on Monday evening.
A search-and-rescue operation is under way, co-ordinated through personnel located at the Hmeimim base, the ministry added.
Earlier, the Israeli military refused to comment on reports its planes targeted facilities in the Latakia area on Monday, saying: "We don't comment on foreign reports."
The Syrian government has not yet commented on the incident.
But reports on Syrian state media talked of an attack in the area shortly before the plane disappeared.
"Our air defences repelled enemy missiles coming from the open sea towards the city of Latakia and intercepted several of them before they could reach their targets," Sana news agency quoted the military as saying late on 17 September.
Syrian television 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.
Hmeimim is Russia's main base for air strikes on rebel groups in Syria - strikes that have enabled President Assad's forces to recover much lost ground since 2015.
Russia says its airstrikes only target "terrorists", but activists have said they mainly hit mainstream rebel fighters and civilians.
According to The Syrian Observatory for Human Rights, a UK-based monitoring group, at least 7,928 civilians and 10,069 combatants have been killed in Russian air strikes.
Russia has also suffered personnel losses, including the deaths of 39 people when a Russian military transport plane crashed as it attempted to land at Hmeimim in March.
Just a few months earlier, Russia said it had increased security at Hmeimim in response to a foiled drone attack on the base just days after reports that rebel shelling had damaged several planes.
Earlier on Monday Russia and Turkey agreed to create a demilitarised buffer zone in Syria's Idlib province to separate government forces from rebel fighters based there.
Russian President Vladimir Putin said it would be 15km to 25km (9-15 miles) wide and come into force by 15 October.