The Greek foreign ministry issued "an immediate and strong-worded protest" to the Turkish ambassador in Athens, saying Ankara was "violating all the fundamental rules of international law".

It said the Turkish fighter jets "illegally entered our national airspace and approached Alexandroupolis within a distance of just 2.5 nautical miles".

"This action constitutes a clear escalation of Turkish provocative conduct. (It) undermines NATO's cohesion and urgent priorities, and poses a clear threat to the European Union at a critical juncture, as the port of Alexandroupolis is a key hub for the transport of troops and equipment to reinforce our allies."

The ministry said it would inform NATO, the EU and the United Nations.

Greece and Turkey are neighbours and allies within the NATO military alliance, but have had strained relations for years, exacerbated recently by Turkish drilling for hydrocarbons in the Mediterranean.

Athens regularly accuses Turkish war planes of violating its airspace