Turkish warplanes have reportedly shot down a Russian military aircraft on the border with Syria.
Russia's Interfax news agency cited the Russian defence ministry as saying that a Su-24 had crashed in northern Syria, and that its two pilots had ejected.
Turkish military officials said Turkish F-16s had shot down the plane after repeatedly warning its pilots that they were violating Turkish airspace.
Video footage showed the plane crashing into mountains in Latakia province.
Russia launched an air campaign to bolster Syrian President Bashar al-Assad's government in late September.
It said it had targeted only "terrorists", but activists say its strikes have mainly hit Western-backed rebel groups.
Turkey, a staunch critic of Mr Assad, 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.