A Russian warplane has been shot down by Turkish jets in the first attack on Russian aircraft by a NATO member since the 1950s.
The Russian aircraft was downed near the Syrian border on Tuesday after repeated warnings over air space violations, but Moscow said it could prove the jet had not left Syrian air space.
It was the first time a NATO member's armed forces have downed a Russian or Soviet military aircraft since the 1950s and Russian and Turkish assets fell on fears of an escalation between the former Cold War enemies.
A Kremlin spokesman said it was a "very serious incident" but that it was too early to draw conclusions.
Footage from private Turkish broadcaster Haberturk TV showed the warplane going down in flames in a woodland area, a long plume of smoke trailing behind it. The plane went down in area known by Turks as "Turkmen Mountain", it said.
Separate footage from Turkey's Anadolu Agency showed two pilots parachuting out of the jet before it crashed. A Syrian rebel group sent a video to Reuters that appeared to show one of the pilots immobile and badly wounded on the ground and an official from the group said he was dead.
Russia's defence ministry said one of its Su-24 fighter jets had been downed in Syria and that, according to preliminary information, the pilots were able to eject. It said the aircraft had been over Syria for the duration of its flight.
The Turkish military said the aircraft had been warned 10 times in the space of five minutes about violating Turkish airspace. Officials said a second plane had also approached the border and been warned.
"The data we have is very clear. There were two planes approaching our border, we warned them as they were getting too close," a senior Turkish official told Reuters.
"We warned them to avoid entering Turkish airspace before they did, and we warned them many times. Our findings show clearly that Turkish airspace was violated multiple times. And they violated it knowingly," the official said.
A second official said the incident was not an action against any specific country but a move to defend Turkey's sovereign territory within its rules of engagement.
Russia's decision to launch separate air strikes in Syria mean Russian and NATO planes have been flying combat missions in the same air space for the first time since World War Two, targeting various insurgent groups close to Turkish borders.
The downing of the jet appeared to scupper hopes of a rapprochement between Russia and the West in the wake of the Islamic State attacks in Paris, which led to calls for a united front against the radical jihadist group in Syria.
Russia's main stock index fell more than two percent, while Turkish stocks fell 1.3 percent. Both the rouble and lira were weaker.
Turkish President Tayyip Erdogan was briefed by the head of the military, while Prime Minister Ahmet Davutoglu ordered consultations with NATO, the United Nations and related countries, their respective offices said.
The Syrian Observatory for Human Rights monitoring group said the warplane crashed in a mountainous area in the northern countryside of Latakia province, where there had been aerial bombardment earlier and where pro-government forces have been battling insurgents on the ground.
"A Russian pilot," a voice is heard saying in the video sent to Reuters as men gather around the man on the ground. "God is great," is also heard.
The rebel group that sent the video operates in the northwestern area of Syria, where groups including the Free Syrian Army are active but Islamic State, which has beheaded captives in the past, has no known presence.
The official from the group, who deciined to be named for security reasons, did not mention the second Russian pilot.
Broadcaster CNN Turk earlier reported that one of the pilots was in the hands of Turkmen forces in Syria who were looking for the other one, citing local sources. Russian military helicopters were also searching for the pilots, Turkey's Dogan news agency said.
Both Russia and its ally, Syria's government, have carried out strikes in the area. A Syrian military source said the reported downing was being investigated.
Turkey called this week for a U.N. Security Council meeting to discuss attacks on Turkmens in neighbouring Syria, and last week Ankara summoned the Russian ambassador to protest the bombing of their villages.
Ankara has traditionally expressed solidarity with Syrian Turkmens, who are Syrians of Turkish descent.
Russian Foreign Minister Sergei Lavrov is due to visit Turkey on Wednesday to discuss Syria, in a trip arranged before this incident. Erdogan is meanwhile expected to visit Russia for talks with Putin in late December.
About 1,700 people have fled the mountainous Syrian area near to the Turkish border as a result of fighting in the last three days, a Turkish official said on Monday. Russian jets have bombed the area in support of ground operations by Syrian government forces.