Envoys to the North Atlantic Treaty Organisation approved the plan and must now decide what military assets to send to Turkey, NATO Secretary-General Jens Stoltenberg said, stressing that it was a defensive measure.

Given that Turkey already has a formidable air force, NATO diplomats and military experts say the alliance's involvement is to minimise the risk of any repeat of Turkey's Nov 24 shooting down of a Russian warplane that flew into Turkish airspace.

That was the first known incident of its kind since the Cold War and the most serious of several air incursions since early October, leaving relations between Turkey and Russia at their lowest ebb in recent memory.

AWACS monitor airspace within a radius of more than 400 km (250 miles) and exchange information via digital data links, with ground-based, sea-based and airborne commanders.

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