A feared incident - Jonathan Marcus, BBC defence correspondent
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.
It suggests that the much discussed arrangements to avoid incidents between warplanes over Syria are inadequate. The Turkish authorities will no doubt claim that such arrangements do not cover the approaches to their own airspace where tried and tested procedures should apply.
There are conflicting reports as to whether it was ground fire or Turkish jets that brought down the Russian plane. Air operations in the crowded skies over Syria just got a good deal more complicated. The only mitigating factor is that initial reports suggest that two parachutes were seen so the Russian crew at least got out of their stricken aircraft. Expect diplomatic fireworks.