Russia said Tuesday one of its Su-24 fighter jets was shot down along the border between Turkey and Syria, but said "firing from the ground" was responsible for downing the plane.
Turkish military officials, however, said their F-16 jets shot down the plane after it violated the country's airspace and ignored 10 warnings in the span of five minutes to leave. Russia's defense ministry said the jet never left Syrian territory.
Video from the area showed what appeared to be two pilots parachuting down from the sky, and a Syrian rebel group later released its own video of what it said was one of the pilots and that he was dead.
Russia said only that its pilots ejected and their status was unknown.
A Kremlin spokesman called it a "very serious incident" but said it was too early to draw conclusions.
NATO is to hold an emergency meeting Tuesday to discuss the incident.
"At the request of Turkey, the North Atlantic Council will hold an extraordinary meeting at 17.00 (Brussels time) (1600 GMT). The aim of this extraordinary NAC meeting is for Turkey to inform Allies about the downing of a Russian airplane," a NATO official said.
The Britain-based Syrian Observatory for Human Rights, which monitors violence in Syria, said the plane went down in the Turkmen Mountains in Latakia province.
Pro-government forces have been fighting in that region and the Islamic State group is not known to be present there.
A U.S. defense official said Turkey informed the United States that its aircraft had shot down a Russian jet that violated its airspace. The official said U.S. forces were not involved.
Turkish officials have issued multiple warnings about aircraft violating its airspace during Syria's 4½-year civil war. Its forces shot down a Syrian helicopter in 2013 and last month took down an unidentified drone that crossed into its territory.
Turkey has also complained about at least two instances of Russian jets flying in its airspace.
NATO protested those incursions, and in October noted what it called the "extreme danger of irresponsible behavior." The alliance is holding an extraordinary meeting to discuss Tuesday's incident at Turkey's request.
After the earlier Russian incursions into Turkey, the United States deployed six F-15 jets from Britain to Turkey's Incirlik Air Base to help the NATO ally secure its skies.
Russia began its military campaign in Syria in late September.
In October, it reached an agreement with the United States, which is leading a coalition of countries bombing militants in Syria, to keep a safe distance and communicate in order to prevent mid-air disasters.
VOA Pentagon correspondent Carla Babb and National Security correspondent Jeff Seldin contributed to this report.
Watch YouTube video of crash from Turkey's state-run Anadolu Ajansi: