Syria accidentally shot down a Russian military aircraft, killing all 15 crew members, when its air defences swung into action against an Israeli missile strike, Russian and Israeli forces said Tuesday.
The incident late Monday was the worst case of friendly fire between the two allies since Russia's game-changing military intervention in September 2015.
The Russian Ilyushin dropped off the radar over the Mediterranean moments after Turkey and Russia announced a deal that offered millions of people reprieve from a threatened military assault in northern Syria.
There was no immediate word from Damascus, but the deadly air war sequence started when Israeli missiles struck the coastal region of Latakia on Monday.
Israel confirmed that it had targeted a Syrian military facility where weapons manufacturing systems were "about to be transferred on behalf of Iran" to Lebanese Shiite group Hezbollah.
It expressed "sorrow" for the Russian deaths, but insisted the Russian plane had been felled by "extensive and inaccurate Syrian anti-aircraft (surface-to-air missile) fire".
The plane downed by Syria's Russian-made S-200 air defence had a crew of 15, who were all killed, Moscow said.
The Russian military claimed Israeli pilots had attacked Syrian targets by using "the Russian plane as a cover, exposing it to fire from Syrian air defences".
But Israel denied the accusations, saying that "when the Syrian army launched the missiles that hit the Russian plane, (Israeli) jets were already within Israeli airspace."
The Russian government stressed that the incident would have no impact on the fledging Idlib deal, but warned Israel of reprisals and summoned its envoy in Moscow.
Syria state media reported late Monday "our air defences are countering hostile missiles coming from the sea towards the city of Latakia, and a number of them have been intercepted," it said, quoting a military source.
Moscow had earlier said rockets were fired from a French frigate positioned in the same area of the Mediterranean.
But the French military denied any involvement, with spokesman Colonel Patrik Steiger telling AFP the frigate Auvergne "did not fire anything last night".
- Israel mum -
The Syrian Observatory for Human Rights said some missiles did get through the Syrian defences and struck ammunition depots at the site of the technical industries institute.
Rami Abdel Rahman, the head of the Britain-based monitoring group, said at least two fighters were killed and 10 wounded in the strike.
"Two bodies were found," he told AFP, adding that he could not immediately confirm whether the casualties were soldiers or militiamen from the forces supporting Syrian President Bashar al-Assad.
Israel has conducted frequent raids in recent months against Syria's military infrastructure, including against bases it believes host Iranian combatants.
Earlier in September, Israel admitted carrying out 200 strikes in Syria over the past 18 months.
The Syrian blunder came hot on the heels of a major deal announced by Russian President Vladimir Putin and his Turkish counterpart Recep Tayyip Erdogan after talks in the Russian resort of Sochi.
The two main brokers in northern Syria agreed to create a 15-20 kilometre-wide demilitarised zone along the line of contact between rebels and regime troops in the Idlib region.
That would be achieved by October 15 and would entail a withdrawal of all jihadist fighters from the area.
The demilitarised zone will be secured with the help of "mobile patrol groups of Turkish contingents and contingents of Russian military police," Putin said.
By the end of the year, transport routes between the key port of Latakia and Aleppo as well as Latakia and the major city of Hama must be restored, he added.
- Idlib reprieve -
Iran, the third member of the so-called Astana process aimed at ending the seven-year civil war in Syria, praised the deal as an example of "responsible diplomacy."
And the European Union said it expected the accord to "guarantee the protection of civilian lives and infrastructure as well as guarantee unhindered and sustainable humanitarian access".
Damascus, which has regained swathes of territory thanks to deadly Russian-backed offensives in recent months, welcomed the Sochi agreement.
Three million people reside in the Idlib region, the last significant chunk of Syria still held by rebels.
The UN had warned that a fully-fledged ground assault aimed at completing Damascus's military reconquest could spark the worst humanitarian catastrophe yet in a conflict that has killed more 360,000.
