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.
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.