ATHENS — The government of Greece slammed Turkey's announcement that it will be conducting energy exploration in an area of the eastern Mediterranean that Athens says overlaps its continental shelf, as tension over the rights to natural resources increased sharply in the region Monday.

Officials said the Greek military was 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 vessels would be conducting seismic exploration in an area between Greece and Cyprus until Aug. 23. 

Last week, Turkey also announced it would be conducing a firing exercise in the sastern Mediterranean 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. 

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

Greek Minister of State Giorgos Gerapetritis said the Oruc Reis was not transmitting through the automatic identification system carried by ships, but 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.

A crucial issue 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 sastern Mediterranean, a move seen as having somewhat defused the situation. 

But Ankara got fired up again last week over a deal Greece and Egypt signed Thursday delineating maritime boundaries and the countries' 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 says it's 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 that the "policy of equal distances is counterproductive and not acceptable."

Late Sunday, Turkish President Recep Tayyip 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.

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." 

Greek PM Mitsotakis speaks with NATO's Stoltenberg

Prime Minister Kyriakos Mitsotakis spoke with NATO Secretary General Jens Stoltenberg on Monday afternoon, briefing the alliance head on developments in the East Mediterranean.

The Greek premier briefed NATO's chief about Turkey's behavior, which undermines stability in the alliance's southeast section, and underlined that the policy of equal distancing is unproductive and unacceptable.

Greek embassy in Ankara filing demarche with Turkey

The Greek embassy in Ankara has been ordered by Foreign Affairs Minister Nikos Dendias to file a strongly worded demarche at the Turkish Foreign Ministry over the presence of the Turkish research vessel "Oruc Reis" in the Greek continental shelf waters, diplomatic sources said on Monday.

Dendias, they said, has also informed fellow ministers of Germany, Cyprus and Bahrein about Turkey's illegal action. He also briefed them about the recent Greece-Egypt exclusive economic zone (EEZ) agreement, which he reiterated follows International Law, the Law of the Sea and rules of good neighborhood, and which prompted Turkey's reaction.