Greece on Monday slammed a Turkish announcement that it will be conducting energy exploration in the eastern Mediterranean in an area Athens says overlaps its continental shelf, as tension increased sharply in the region.

Officials said Greece's 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, announcing its research vessel Oruc Reis and two auxiliary vessels would be conducting seismic exploration in an area between Greece and Cyprus from Monday until Aug. 23.

Last week, Turkey also announced it would be conducing a firing exercise in the eastern Mediterranean this 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 the 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 Monday that 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 on seismic research had been issued by an ``unauthorized station'' and referred to ``unauthorized and illegal activity in an area that overlaps the Greek 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 been high in the region 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, and the move was seen as somewhat defusing the situation.

But last week Ankara slammed a deal signed on Thursday between Greece and Egypt 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, who all said it 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.

NATO allies and neighbors Greece and Turkey have been at odds for decades over a wide variety of issues, including sea boundaries, 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 a spike in tension.

Mitsotakis spoke Monday morning with European Council President Charles Michel, informing him about the Greek-Egyptian agreement and the situation in the eastern Mediterranean, officials said. He was scheduled to speak to NATO Secretary General Jens Stoltenberg on Monday afternoon.

In a television interview late Sunday, Turkish President Recep Tayyip Erdogan's spokesman, Ibrahim Kalin said Turkey and Greece had been holding talks in Berlin for 2{ 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.