The Turkish military has reportedly shot down a Russian military aircraft on the border with Syria.
Russia's defence ministry said an Su-24 had crashed on Syrian territory after being hit by fire from the ground, and that its pilots had managed to eject.
But Turkish military officials said Turkish F-16s had shot down the plane after repeatedly warning its pilots they were violating Turkish airspace, BBC reports.
Video footage showed the plane crashing into mountains in Latakia province.
It is believed to have come down in a rebel-held area.
Russia launched an air campaign against opponents of Syrian President Bashar al-Assad's government in late September.
Türkmen Dağı'nın yakınına savaş uçağı düştü...
Uçağın düşme anını Habertürk TV ekibi görüntüledi24 ноября 2015
The Russian defence ministry confirmed on Tuesday morning that a Russian Su-24 had "crashed on Syrian territory, having been hit from the ground" while it was flying at an altitude of 6,000m (19,685ft).
"Efforts are being made to ascertain what happened to the pilots. According to preliminary reports, the pilots have managed to self-eject," the ministry was quoted as saying by the Interfax news agency.
The ministry stressed that "throughout its flight, the aircraft remained exclusively above Syrian territory", adding: "Objective monitoring data shows it."
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, Turkish military officials said its F-16s had fired on an unidentified aircraft after warning it 10 times in the space of five minutes about violating Turkish airspace.
The UK-based Syrian Observatory for Human Rights said the Russian 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.
Qatar-based Al Jazeera TV quoted an ethnic Turkmen rebel group as saying it had captured one of the Su-24's two pilots and were "looking for the other".
Russian military helicopters were searching for the pilots near the crash site, Turkey's Dogan news agency reported.
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 Latakia province.
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.