Exclusive look inside Nato's military nerve centre

On Friday, the US Secretary of State John Kerry meets the Russian foreign minister in Vienna to discuss Syria.

But the conflict in the Middle East and last year in Ukraine has brought relations between Russia and the West to their lowest ebb since the Cold War.

It has also put NATO allies on high alert - in particular Turkey, as it scans the skies for incursions by Russian planes.

The BBC's Defence Correspondent Jonathan Beale reports from southern Europe.