Turkish warplanes have shot down a Russian military aircraft on the border with Syria.
Russian President Vladimir Putin said the Su-24 was hit by air-to-air missiles fired by Turkish F-16s while it was flying over Syrian territory.
But Turkish military officials said the plane was engaged after being warned that it was violating Turkish airspace.
Mr Putin described the incident as a "stab in the back" committed by "accomplices of terrorists".
The crew ejected before the jet crashed in Latakia province, but Syrian rebels said at least one was dead.
It is the first time a Russian aircraft has crashed in Syria since Moscow launched air strikes against opponents of President Bashar al-Assad in late September.
The Nato military alliance, to which Turkey belongs, said it was following the situation "closely" and was in contact with the Turkish authorities. There will be an "informational meeting" of ambassadors in Brussels at 16:00 GMT.
'Exclusively above Syria'
At a meeting with King Abdullah II of Jordan in Sochi, Russia's president confirmed that the Su-24 had been shot down over Syrian territory, 1km (0.6 miles) from the Turkish border, by an air-to-air missile from a Turkish F-16 jet.
It crashed in Syrian territory 4km (2.5 miles) from the border, he added.
"This goes beyond the normal struggle against terrorism. This was a stab in the back by the accomplices of terrorists," Mr Putin stated, an apparent reference to Turkey's support for Syrian rebel groups.
"Our pilots and our plane did not in any way threaten Turkey. It is quite clear," he added.
"They were carrying out an operation against [Islamic State militants] in the mountains of northern Latakia, where militants who originate from Russian territory are concentrated. So they were carrying the key task of preventative attacks against those who could return to Russia at any time."
A feared incident - Jonathan Marcus, BBC defence correspondent
This is exactly the kind of incident that many have feared since Russia launched its air operations in Syria. The dangers of operating near to the Turkish border have been all too apparent. Turkish planes have already shot down at least one Syrian air force jet and possibly a helicopter as well.
Russia insists that its warplane did not violate Turkish air space. So, was the Russian pilot's navigation wrong? Questions will also be asked about the readiness of the Turks to open fire.
It suggests that the much discussed arrangements to avoid incidents between warplanes over Syria are inadequate. The Turkish authorities will no doubt claim that such arrangements do not cover the approaches to their own airspace where tried and tested procedures should apply.
There are conflicting reports as to whether it was ground fire or Turkish jets that brought down the Russian plane. Air operations in the crowded skies over Syria just got a good deal more complicated. The only mitigating factor is that initial reports suggest that two parachutes were seen so the Russian crew at least got out of their stricken aircraft. Expect diplomatic fireworks.
However, the Turkish military said two F-16s on patrol had fired on an unidentified aircraft at 09:24 (07:24 GMT) after warning it 10 times over five minutes about violating Turkish airspace over the town of Yayladagi, in Hatay province.
It noted that the F-16s had intervened "in accordance with the rules of engagement", which were changed after Syria shot down a Turkish plane in 2012.
The UK-based Syrian Observatory for Human Rights said the jet had crashed in the mountainous Jabal Turkmen area of Latakia, where air strikes and fighting between rebels and Syrian government forces had been reported earlier on Tuesday.
The Syrian Observatory also reported that a Russian military helicopter had been forced to make an emergency landing after being hit by rebel fire near the crash site.
A spokesman for a rebel group operating in the area, the 10th Brigade of the Coast, told the Associated Press that the jet's crew had tried to parachute into government-held territory, but that they came under fire from members of the group.
One of them was dead when he landed on the ground, he added. The fate of the second was not immediately known.
A video was posted online showing gunmen standing around a man in a flight suit who was immobile on the ground, either badly wounded or dead.
Russian aircraft have flown hundreds of sorties over northern Syria since September. Moscow says they have targeted only "terrorists", but activists say its strikes have mainly hit Western-backed rebel groups.
Turkey, a vehement opponent of Syria's president, has warned against violations of its airspace by Russian and Syrian aircraft.
Last month, Ankara said Turkish F-16s had intercepted a Russian jet that crossed its border and two Turkish jets had been harassed by an unidentified Mig-29.
The Turkish foreign ministry also summoned the Russian ambassador last week to warn him that there would be "serious consequences" if the Russian air force did not immediately stop bombing "civilian Turkmen villages" in the Bayir Bucak area, near Tuesday's crash site.
In a separate development on Tuesday, three Russian journalists were lightly wounded on Monday while driving in a convoy towards the Syrian government army frontline near Dagmashliya, in north-western Syria. Their vehicles came under fire, apparently from TOW anti-tank missiles.
Russian media named them as Tass correspondent Alexander Yelistratov, Russia Today Arabic Service TV correspondent Sargon Khadaya and RT English correspondent Roman Kosarev.