ATHENS, Greece (AP) — Greece slammed Turkey’s move Monday to launch energy exploration in part of the eastern Mediterranean Athens says overlaps its own continental shelf, as regional tension spiked over rights to offshore natural resources with both countries sending warships to the area.

The Greek military was placed on alert, while Prime Minister Kyriakos Mitsotakis convened the government’s national security council after Turkey issued a Navtex, or international maritime safety message, Monday announcing its research vessel Oruc Reis and two auxiliary ships would be conducting seismic exploration in an area between Greece and Cyprus until Aug. 23.

Turkish Energy and Natural Resources Minister Fatih Donmez said the Oruc Reis had arrived in its area of operation Monday morning from its anchorage off Turkey’s southern coast. He tweeted that “83 million back the Oruc Reis,” referring to Turkey’s population.

Turkey’s Defense Ministry said naval vessels were escorting the Oruc Reis to “protect our rights,” and tweeted images of the vessel flanked by five warships.

Last week, Turkey also announced it would be conducing a firing exercise Monday and Tuesday in a nearby area, southwest of the Turkish coast between Turkey and the Greek island of Rhodes.

“Greece will not accept any blackmail. It will defend its sovereignty and sovereign rights,” Greece’s Foreign Ministry said in a statement. “We call on Turkey to immediately end its illegal actions that undermine peace and security in the region.”

The ministry said Monday’s Navtex “combined with the observed broad mobilization of units of the Turkish Navy, constitutes a new serious escalation.” Turkey is acting in a way that is destabilizing and threatening peace, it added.

Greek Minister of State Giorgos Gerapetritis said the Oruc Reis was being monitored by the Greek navy.

“We are at full political and operational readiness,” Gerapetritis said on state television ERT.

“The majority of the fleet is ready at this moment to go out wherever is needed,” he said when asked to elaborate. “Our ships that are sailing in crucial areas were already in place days ago. If necessary, there will be a greater development of the fleet.”

Gerapetritis said that “it is clear that we are not seeking any tension in the region. On the other hand, our determination is a given.”

Greece on Monday issued its own maritime safety message saying the Turkish Navtex had been issued by an “unauthorized station” and referring to “unauthorized and illegal activity in an area that overlaps the Greek continental shelf.”

Turkey retorted with another maritime message saying the seismic survey was being conducted on Turkey’s continental shelf.

Speaking after a four-hour Cabinet meeting, Turkish President Recep Tayyip Erdogan warned that Turkey would not confine its offshore exploration efforts to its immediate coastline, but otherwise appeared conciliatory.

“Let’s come together as Mediterranean countries. Let’s find a formula that’s acceptable for everyone, that protects everyone’s rights,” he said in a televised speech.

He added: “We are always there and ready for the solution of disputes through dialogue and on a fair basis. We will continue to implement our own plans in the (eastern Mediterranean) and in the field of diplomacy until common sense prevails in this regard.”

At the crux of the dispute is whether islands should be included in calculating a country’s continental shelf and maritime zones of economic interest. Turkey argues they should not be, a position Greece says violates international law. Greece has thousands of islands and islets in the Aegean and Ionian seas, around 200 of them inhabited.

Tension has increased in recent months over drilling rights and maritime boundaries. Late last month, Turkey had said it was suspending its exploratory operations in the eastern Mediterranean, a move seen as having somewhat defused the situation.

But Ankara was angered by a deal Greece and Egypt signed Thursday delineating their maritime boundaries and exclusive economic zones for drilling rights.

Last year, Turkey signed a similar deal with the U.N.-backed Libyan government in Tripoli, sparking outrage in Greece, Cyprus and Egypt, which all said the agreement infringed on their economic rights in the Mediterranean. The European Union said the deal was a violation of intentional law that threatens stability in the region.

German Foreign Ministry spokesman Christofer Burger said Berlin had “taken note with concern” of Turkey’s decision to conduct seismic exploration. He said Foreign Minister Heiko Maas “has repeatedly said that international law must be respected and that we need steps toward deescalation in the Eastern Mediterranean. And in view of this, further seismic exploration is certainly the wrong signal at this time.”

Turkey’s move “further burdens its relationship with the EU,” Burger added. He called on both sides “to resolve all open questions through negotiations and to begin a bilateral dialogue between Athens and Ankara as planned.”

NATO allies Greece and Turkey have been at odds for decades over a wide variety of issues and have come to the brink of war three times since the mid-1970s, including once over drilling exploration rights. Recent discoveries of natural gas and drilling plans across the east Mediterranean have led to renewed tension.

Greece’s prime minister Mitsotakis spoke Monday with European Council President Charles Michel and NATO Secretary General Jens Stoltenberg.

After the call, Stoltenberg tweeted that “the situation must be resolved in a spirit of Allied solidarity and in accordance with international law.”

A Greek government official, speaking on condition of anonymity to discuss the contents of the conversation, said Mitsotakis stressed during the call that the “policy of equal distances is counterproductive and not acceptable.”

Late Sunday, Erdogan’s spokesman, Ibrahim Kalin said Turkey and Greece had been holding talks in Berlin for 2½ months and were on the verge of issuing a joint statement when the Greek-Egyptian agreement emerged.

“The moment the agreement with Egypt was announced, we received a clear instruction from our president: ‘You are halting the talks. Inform the Germans and the Greeks, we are not pressing ahead with the negotiations,’” Kalin told CNN-Turk television. “This is another move to keep Turkey out of the Eastern Mediterranean and to restrict it to the Gulf of Antalya.”

Kalin said Turkey is in favor of resolving the dispute through dialogue, “but it is the Greek side that disrupted the agreement and broke the trust.”

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Andrew Wilks and Suzan Fraser in Ankara contributed to this report.