Russian President Vladimir Putin has condemned the downing of a Russian fighter jet by Turkey as a stab in the back administered by "the accomplices of terrorists".
Turkey says its jets shot at the plane after warning multiple times that it was straying into Turkish airspace. But Moscow says it never strayed from Syrian airspace.
Mr Putin said the downed plane had been attacked inside Syria when it was 1km from the Turkish border and had come down 4km inside Syria.
The Russian president said the incident would have serious consequences for Moscow's relations with Ankara.
"Today's loss is linked to a stab in the back delivered to us by accomplices of terrorists. I cannot qualify what happened today as anything else."
He said Russian pilots and planes had in no way threatened Turkey, but had merely been carrying out their duty to fight Islamic State militants inside Syria.
"We will of course analyse everything that happened and today's tragic events will have serious consequences for Russo-Turkish relations," he said.
Turkey told the UN Security Council on Tuesday it had shot down on an unidentified plane that violated Turkish airspace and defended its right to do so.
In a letter, Turkish UN Ambassador Halit Cevik said two planes approached Turkish airspace on Tuesday morning and were warned 10 times in five minutes to change direction.
Mr Cevik said both planes then flew more than a mile into Turkey for 17 seconds. He said the nationality of the planes was unknown.
NATO Secretary-General Jens Stoltenberg offered support for Turkey and rejected any suggestion that the incident happened outside of Turkey's borders.
Asked if there was any possibility that the plane was actually shot down inside Syria, Mr Stoltenberg said the allied assessments were consistent with the information from Turkey.
"We stand in solidarity with Turkey and support the territorial integrity of our NATO ally," he said.
Fate of pilots unclear
Footage from Turkey's Anadolu Agency showed the jet's two pilots parachuting out of the plane before it crashed.
A deputy commander of rebel Turkmen forces in Syria said his men shot both pilots dead as they came down.
But a Turkish government official told Reuters the pilots were believed still to be alive and that Ankara was working to secure their release from Syrian rebels.
- Reuters / BBC