Greece on Monday accused Turkey of "threatening peace" in the eastern Mediterranean and called a military meeting after Ankara resumed searching for oil and gas near a Greek island.
The Greek foreign ministry said that Turkey's decision to deploy seismic research ship Oruc Reis constituted a "new serious escalation" and "exposed" Turkey's "destabilising role".
Energy exploration in the gas-rich eastern Mediterranean is a frequent source of tension between Turkey and a bloc of neighbours including Greece, Cyprus, and Israel.
The Greek ministry said Athens "will not accept any blackmail" and "will defend its sovereignty and its sovereign rights".
The announcement came after Greek Prime Minister Kyriakos Mitsotakis conferred with military chiefs and his foreign minister.
Mitsotakis' office said the PM had spoken to EU Council President Charles Michel and NATO Secretary General Jens Stoltenberg.
"I spoke with Greek Prime Minister (Mitsotakis) today on the situation in the eastern Mediterranean. The situation must be resolved in a spirit of Allied solidarity and in accordance with international law," Stoltenberg later tweeted.
The Oruc Reis has reached the eastermost part of Greece's maritime jurisdiction, state agency ANA reported.
A senior Greek minister added that navy ships were monitoring the Turkish seismic research ship.
"We are in complete political and operational readiness," Minister of State George Gerapetritis said on state TV ERT.
"Most of the fleet is ready to be deployed wherever necessary," he said.
Turkish Energy Minister Fatih Donmez had earlier tweeted that the Oruc Reis had "reached the destination where work would be undertaken", near the island of Kastellorizo.
Turkey sent out a message on NAVTEX, the international maritime navigational telex system, announcing the vessel would be carrying out activities off the island of Kastellorizo between August 10 and 23.
The move came just days after the NATO allies seemed close to talks over disputed maritime zones in the Aegean.
Turkey had called off an earlier search by the Oruc Reis last month to hold negotiations with Greece and Germany, which holds the rotating EU presidency.
But the mood soured last week after Greece and Egypt signed an agreement to set up an exclusive economic zone in the region.
The Turkish foreign ministry said the "so-called maritime deal" was "null and void".
Egypt, Cyprus and Greece have likewise denounced a contentious deal, including a security agreement, signed last year between Ankara and UN-recognised government in Libya.
Greece, Cyprus and Israel in January signed an agreement for a huge pipeline project to transport gas from the eastern Mediterranean to Europe despite Turkey's hostility to the deal.