Turkey and Greece announced that they will conduct rival naval exercises off the Greek island of Crete on Tuesday amid growing tension over gas and oil claims in the eastern Mediterranean.
Turkey has sent an official warning to other vessels to avoid the area.
Greece announced its exercises after Turkey said it would extend a mission by a seismic research vessel.
Germany is sending its Foreign Minister Heiko Maas to Athens and Ankara on Tuesday to try to reduce tensions.
Maas is due to meet Greek Prime Minister Kyriakos Mitsotakis before meeting in Ankara with his Turkish counterpart.
Turkey and Greece – both members of NATO – are at odds over the discovery of oil and gas fields off Crete and Cyprus in disputed waters.
Greece is part of the European Union, which called for dialogue. But France appears to be rallying behind Greece – it was involved in a recent naval exercise with Greece.
Turkey announced Monday that exploration by its research vessel Oruc Reis in the disputed waters would be extended for four days until August 27. This appears to have prompted Greece, which considers the investigation illegal, to report the naval exercises.
“Greece is responding calmly and eagerly at both diplomatic and operational levels. And with national confidence, it does whatever is necessary to defend its sovereign rights, ”said Greek government spokesman Stelios Petsas.
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Turkey responded in an equally robust manner.
“Turkey will not take the slightest step back from the activities of Oruc Reis or our naval elements which escort him,” said President Recip Tayyip Erdogan.
Greece has “thrown itself into chaos from which it cannot find a way out,” he declared.
Athens reacted angrily last month when Ankara issued a similar advisory, known as Navtex, warning of the presence of its research vessel.
Greece signed a maritime border agreement with Egypt this month to declare exclusive economic zones, but the agreement overlaps with the one signed by Turkey and the UN-recognized Libyan government in Tripoli.
To further complicate matters, Greek officials said on Friday that the United Arab Emirates (UAE) would send F-16 fighter jets to Crete for joint training this week.
Relations between Turkey and Greece have deteriorated several times over the past decades in a frightening dispute over the Aegean islands. On three occasions, tensions escalated near the point of conflict.
Separately, Turkey announced last week that a drillship had discovered a large reserve of natural gas in the Black Sea, its biggest Turkish discovery to date.