ATHENS, Greece (AP) — Greece accused neighbor Turkey of undermining efforts to ease a crisis over eastern Mediterranean drilling rights Monday, after Ankara redeployed a survey vessel for new energy exploration in disputed waters — including an area very close to a secluded Greek island.

The move reignited tension over sea boundaries between Greek islands, Cyprus and Turkey’s southern coast which had flared up over the summer, prompting a military build-up, bellicose rhetoric and fears of a confrontation between the two NATO members and historic regional rivals.

The Turkish search vessel, Oruc Reis, left the port of Antalya on Monday. An international maritime safety advisory, or Navtex, issued late Sunday said the exploration would last until Oct. 22.

Turkish Defense Minister Hulusi Akar said the vessel was continuing with its “planned and scheduled activities,” adding that the Turkish navy would provide “support and protection” if necessary.

Greek Prime Minister Kyriakos Mitsotakis discussed the development on the phone with European Council President Charles Michel, saying he would bring it up at the next council meeting on Oct. 15-16.

“This new unilateral act is a severe escalation on Turkey’s part,” a government statement quoted Mitsotakis as saying.

On Monday, Turkey said Greek objections to Orus Reis' redeployment were “unacceptable,” insisting that the search vessel was operating within Turkey's continental shelf — in area just 15 kilometers (nearly 10 miles) away from the Turkish coast and 425 kilometers (about 265 miles) away from mainland Greece.

“Our expectation from Greece is for it to withdraw its maximalist claims that are contrary to international law ... put an end to its exercises and military activities that increase tensions in the Aegean and the Mediterranean and to enter into a sincere dialogue with us,” the Turkish Foreign Ministry said.

Turkey faces the threat of sanctions from the European Union, which has sided in the dispute with member states Greece and Cyprus.

German Foreign Minister Heiko Maas, whose country holds the rotating EU presidency and has been mediating between Ankara and Athens, will fly to Cyprus and Greece for talks on Tuesday.

On Monday, the German government said it had “taken note” of Turkey’s announcement on the energy prospecting.

“If there really were exploration in this disputed area of sea, that would be a very regrettable step and, from our point of view, an unwise one,” German Chancellor Angela Merkel’s spokesman, Steffen Seibert, told reporters in Berlin. “It would set back efforts to reduce tensions in the eastern Mediterranean and it most certainly would be anything but conducive to the continued development of EU-Turkish relations.”

Seibert reiterated Germany’s insistence that “it is important and necessary for all involved to make an effort to prevent escalations and to resolve their differences in the eastern Mediterranean — including the differences on maritime law — as quickly as possible, in dialogue and on the basis of international law.”

He said Germany remains ready to support efforts to that end.

Greek Foreign Minister Nikos Dendias said he briefed his EU colleagues on Turkey’s planned “illegal actions” within Greece’s continental shelf south of Kastellorizo, a Greek islet just off Turkey’s southern coast.

“I explained the obvious, who is the common denominator in all problematic situations in the area: Nagorno-Karabakh, Syria, Iraq, Cyprus, the southeastern Mediterranean. The common denominator is Turkey,” Dendias said — referring to regional hot spots in which Ankara has a military presence or active diplomatic involvement. “Turkey is undermining peace and stability in the region.”

Ankara and Athens had earlier this month agreed, under NATO’s auspices, to set up a system to avoid potential military conflicts and accidents, including a hotline. The two countries had also agreed to resume so-called exploratory talks aimed at building confidence and resolving disputes which were last held in 2016.

The Turkish vessel’s return comes a day after Turkey’s Foreign Ministry issued a statement accusing Greece of “insincerity” for what it said was Athens’ continued actions to raise tension while declaring itself to be ready for a dialogue. Among other things, the Turkish Foreign Ministry accused Greece of declaring military exercises in the Aegean Sea to coincide with Turkey’s Oct. 29 national day celebrations. Turkey retaliated by declaring exercises on Oct. 28 — a Greek national holiday — the statement said.

Energy Minister Fatih Donmez tweeted that the Orus Reis had raised its anchor to “take the X-ray” of the Mediterranean seabed, after the completion of maintenance work.

Turkey had announced last month that it was pulling the Orus Reis to shore for maintenance and resupply, saying the move would give “diplomacy a chance.”

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Suzan Fraser reported from Ankara, Turkey. Derek Gatopoulos in Athens, and Geir Moulson in Berlin contributed to this report.