Turkey has sent a ship to conduct a seismic survey in a disputed area of the eastern Mediterranean – a move that has put Greece on the alert.
The search for potentially rich oil and gas depots will take place south of the Greek island of Kastellorizo.
Greek Prime Minister Kyriakos Mitsotakis called a meeting with military commanders on Monday, after Turkey issued an international maritime warning, known as a Navtex.
The mission of the Oruc Journey challenges a Greek-Egyptian gas exploration bond.
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There are also tensions around Cyprus over rival exploration rights. The Republic of Cyprus and Greece do not accept such rights for Turkish-controlled northern Cyprus in the region.
The self-styled Turkish Republic of Northern Cyprus, defended by Turkey, is not internationally recognized. Last year, Turkey sent two drilling vessels – the Fatih and Yavuz – to waters outside Cyprus.
The Oruc Journey is accompanied by two helpers. The trip was suspended by Turkey last month, thanks to international concerns.
The Greek Foreign Ministry said the latest mission was a “new serious escalation” that “exposed” Turkey’s “destabilizing role”.
Greece and Turkey are both NATO members, but have a history of border disputes.
Last Thursday, Greece and Egypt signed a deal to create an exclusive economic zone (EEZ) in waters containing oil and gas reserves.
The deal conflicts with a rival EEZ that Turkey and the internationally recognized government in Libya have sought to establish. Such zones can extend for 200 nautical miles.
The Republic of Cyprus has EEZ agreement with Lebanon, Egypt and Israel, directly linked to the development of offshore energy resources.