Turkey and Greece have announced they will conduct rival naval exercises off the Greek island of Crete on Tuesday amid rising tension over claims to gas and oil in the Eastern Mediterranean.
Turkey sent out 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 ship.
Germany is sending its Foreign Minister Heiko Maas to Athens and Ankara on Tuesday to try to reduce tension.
Mr Maas is due to meet Greek Prime Minister Kyriakos Mitsotakis before talks in Ankara with his Turkish counterpart.
Turkey and Greece – both Nato members – are at loggerheads over the discovery of oil and gas deposits off Crete and Cyprus in disputed waters.
Greece is in the European Union, which has called for dialogue. But France appears to side with Greece – it has been involved in a recent naval exercise with Greece.
Turkey announced on Monday that exploration by its Oruc Reis research vessel in contentious waters would be extended for four days till 27 August. That appears to have spurred Greece, which sees the survey as unlawful, to signal naval exercises.
“Greece is responding calmly and with readiness both on a diplomatic and on an operational level. And with national confidence it does everything needed to defend its sovereign rights,” Greek government spokesman Stelios Petsas said.
Turkey responded in a similarly robust fashion.
“Turkey will not take even the smallest step back from the activities of either Oruc Reis or our naval elements escorting it,” President Recip Tayyip Erdogan said.
Greece had “thrown itself into a chaos from which it cannot find a way out,” he said.
Athens reacted angrily last month when Ankara issued a similar advisory, known as a Navtex, warning of the presence of its research ship.
Greece signed a maritime border deal with Egypt this month to declare exclusive economic zones, but that agreement impinges on one signed by Turkey and the UN-recognised Libyan government in Tripoli.
Further complicating matters, Greek officials said on Friday that the United Arab Emirates (UAE) would dispatch F-16 fighter jets to Crete for joint training this week.
Relations between Turkey and Greece have deteriorated several times in the past few decades in a festering dispute over islands in the Aegean. On three occasions tensions have escalated close to the point of conflict.