Tensions spiked between Greece and Turkey on Monday over offshore rights in the eastern Mediterranean, with Athens vowing to push its EU peers to activate sanctions against Ankara.
The deterioration in relations followed the departure earlier on Monday of a Turkish ship, the Oruc Reis, to conduct work south of the Greek island of Kastellorizo, which is close to Turkey's southern coast.
Greece's foreign ministry described the development as a "major escalation" and a "direct threat to peace in the region".
Ankara had withdrawn the same vessel from contested waters in the eastern Mediterranean last month to "allow for diplomacy" before an EU summit at which Cyprus pushed for sanctions against Turkey.
"Turkey has proven it lacks credibility. All those who believed Turkey meant all it said before the European summit of Oct. 1-2 now stand corrected," Greek government spokesman Stelios Petsas said on Monday.
Turkish Energy Minister Fatih Donmez wrote on Twitter that the Oruc Reis had weighed anchor after undergoing maintenance.
"We will continue to explore, dig and protect our rights," he wrote.
Relations between Greece and Turkey are complicated by a range of disputes, from jurisdiction in the Mediterranean to ethnically split Cyprus.
The Cyprus situation was inflamed last week by the Turkish Cypriot north partially reopening a beach resort abandoned by Greek Cypriot residents during a 1974 war.
In a sign of the growing tensions, about 200 people brandishing a large Greek flag burst through a checkpoint close to Varosha in eastern Cyprus on Sunday night, attempting to cross to the Turkish Cypriot side of the island.
Video footage showed them pushing through a thinly-staffed police cordon leading to a United Nations controlled buffer zone, lighting flares and chanting "Cyprus is Greek".
At the summit in early October, the EU said it would punish Turkey if it continued operations in the region and that sanctions could be imposed as soon as December. Ankara said such a warning strained ties.
Greek government spokesman Petsas said the EU does not need to wait two months before taking action.
"Turkey did the precise opposite to what was sought by the European Council ... So the only issue here is to activate more drastic solutions, for Turkey to feel more stick and less carrot this time," Petsas said.
(Reporting by Tuvan Gumrukcu in Ankara, Yesim Dikmen in Istanbul and George Georgiopoulos in Athens Writing by Ezgi Erkoyun and Daren Butler Editing by Timothy Heritage and David Goodman)