The Turkish ship at the center of a row with the European Union over offshore exploration rights set sail on Monday to conduct new seismic surveys in the Mediterranean, a move that Athens called a "major escalation" threatening regional peace.
The Oruc Reis plans to conduct work south of the Greek island of Kastellorizo, which is close to Turkey's southern coast, according to a naval advisory issued late on Sunday.
Tensions between Turkey and Greece appear to have risen after a brief calm in which the sides agreed to resume talks that would include discussion of overlapping claims to potential maritime energy resources.
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.
Last month, Ankara withdrew the vessel from contested waters in the Eastern Mediterranean to "allow for diplomacy" before an EU summit at which Cyprus had pushed for sanctions and Turkey.
The EU said at the summit it would punish Turkey if it continued its operations in the region and that sanctions could be imposed as soon as December. Ankara said such a warning strained ties.
Shipping data on Monday showed the Oruc Reis heading south from the port of Antalya. Two other vessels, the Ataman and Cengiz Han will work in an area including to the south of Kastellorizo until Oct. 22, the naval NAVTEX notice said.
"The new Turkish NAVTEX on surveys south of Kastellorizo within the Greek continental shelf, at a distance of just 6.5 nautical miles from Greek shores, is a major escalation," Greece's foreign ministry said.
The move, after Ankara committed to proposing a date for exploratory talks, showed Turkey is "unreliable" and "does not really truly want a dialog," it said.