NATO member Turkey on Tuesday shot down a Russian war plane on the Syrian border, an act President Vladimir Putin said would have "serious consequences" for ties between two key protagonists in the Syria war.
The Turkish army said the plane was shot down by two F-16s after violating Turkish airspace 10 times within a five-minute period, an account challenged by Moscow which said it was over Syria.
Turkish television pictures showed the jet exploding and crashing in a ball of flames into a Syrian mountain.
Turkish media said one pilot had been captured by rebel forces in Syria after both ejected by parachute while Syrian opposition sources said one was dead and another missing.
NATO has called an emergency meeting over the incident, the first of its kind since Russia launched air strikes in Syria in September, to the consternation of the West.
The presence of military aircraft from Russia, the United States, France, Turkey and a clutch of Gulf states in Syrian skies had long raised fears of an incident that could quickly escalate into a major diplomatic and military crisis.
With a major diplomatic crisis looming between two states on opposing sides in the Syria conflict, Russia angrily insisted its jet never had entered Turkish airspace.
The shooting down of the plane was "a stab in the back committed by accomplices of terrorists," Putin said at a meeting with Jordanian King Abdullah II in Moscow.
Putin said the plane fell in Syrian territory four kilometres (2.5 miles) from the border with Turkey and "did not in any way threaten Turkey".
"Today's tragic event will have serious consequences for Russian-Turkish relations," he warned.
The Turkish army said the downing took place over the Yayladagi district of Turkey's Hatay province on the border with Syria.
"The plane violated Turkish air space 10 times in five minutes despite warnings," the army said in a statement, adding it was shot down at 0724 GMT "according to the rules of engagement".
Russia summoned the Turkish military attache in Moscow while Ankara summoned Moscow's charge d'affaires to the foreign ministry.
"Everyone must know that it is our international right and national duty to take any measure against whoever violates our air or land borders," Turkish Prime Minister Ahmet Davutoglu said.
Reports said two pilots had ejected from the plane and Turkish television pictures showed two white parachutes descending to the ground. Their fate was not certain.
CNN-Turk said Syrian Turkmen forces fighting the Russian-backed regime of President Bashar al-Assad had captured one pilot.
Syrian opposition sources meanwhile told AFP one pilot was dead, the second missing.
Turkey's Dogan news agency broadcast footage of what it said was Russian helicopters flying over Syrian territory in an apparent search for the lost men.
The incident came as Russian and Syrian jets are waging a heavy bombing campaign against targets in northern Syria while the US-led coalition continues its own air strikes.
Turkey has expressed anger at the operation, saying it is aimed at buttressing the Syrian regime and has displaced thousands of Turkmen Syrians, an ethnic minority in the area and strong allies of Ankara.
Russia however insists its air strikes are aimed against ISIS jihadists, who are also being targeted by the US-led coalition.
At Ankara's request, NATO allies will hold an "extraordinary" meeting at 1600 GMT to discuss the incident, an alliance official said.
"NATO is monitoring the situation closely. We are in contact with Turkish authorities."
Russian fighter jets entered Turkish airspace in two separate incidents in October, prompting Ankara to summon the Russian ambassador twice in protest.
Turkey and Russia have long been at loggerheads over the Syrian conflict, with Ankara seeking Assad's overthrow while Moscow does everything to keep him in power.
The Turkish military in October also shot down a Russian-made drone that had entered its airspace. But Moscow denied the drone belonged to its forces.
It remains to be seen what action Turkey could call for at NATO.
Turkey in July invoked NATO's rarely-used article four -- which allows any member to request a meeting of all 28 NATO ambassadors -- over its campaign against Kurdish rebels.
Russian Foreign Minister Sergei Lavrov is due to visit Turkey on Wednesday in a bid to smooth ties and find a joint approach to finding peace in Syria.
Along with Saudi Arabia and the United States, Turkey and Russia are taking part in talks in Vienna that aim to narrow differences on the Syria conflict and have taken on an extra importance after the Paris attacks.
A Turkish foreign ministry official told AFP Lavrov's visit would go ahead as planned: "There is no change in the programme."