Turkish warplanes have shot down a Russian military aircraft on the border with Syria.
Russian President Vladimir Putin said the Su-24 was hit by air-to-air missiles fired by Turkish F-16s while it was flying over Syrian territory.
But Turkish military officials said the plane was engaged after being warned that it was violating Turkish airspace.
Mr Putin described the incident as a "stab in the back" committed by "accomplices of terrorists".
The Nato military alliance, to which Turkey belongs, said it was following the situation "closely" and was in contact with the Turkish authorities. There will be an "informational meeting" of ambassadors in Brussels at 16:00 GMT.
'Exclusively above Syria'
At a meeting with King Abdullah II of Jordan in Sochi, Russia's president confirmed that the Su-24 had been shot down over Syrian territory, 1km (0.6 miles) from the Turkish border, by an air-to-air missile from a Turkish F-16 jet.
It crashed in Syrian territory 4km (2.5 miles) from the border, he added.
"This goes beyond the normal struggle against terrorism. This was a stab in the back by the accomplices of terrorists," Mr Putin stated, an apparent reference to Turkey's support for Syrian rebel groups.
"Our pilots and our plane did not in any way threaten Turkey. It is quite clear," he added.
"They were carrying out an operation against [Islamic State militants] in the mountains of northern Latakia, where militants who originate from Russian territory are concentrated. So they were carrying the key task of preventative attacks against those who could return to Russia at any time."
This is exactly the kind of incident that many have feared since Russia launched its air operations in Syria. The dangers of operating near to the Turkish border have been all too apparent. Turkish planes have already shot down at least one Syrian air force jet and possibly a helicopter as well.
Russia insists that its warplane did not violate Turkish air space. So, was the Russian pilot's navigation wrong? Questions will also be asked about the readiness of the Turks to open fire.