President Recep Tayyip Erdogan warned on Thursday of a "heavy price" if a Turkish vessel searching for gas in disputed Mediterranean waters was attacked and hinted that one incident had already occurred.
Existing tensions between uneasy NATO allies Turkey and Greece escalated when Ankara sent a seismic research ship named Oruc Reis to explore off the Greek island of Kastellorizo on Monday.
Several of Turkey's small Navy ships escorted the Oruc Reis while its helicopters patrolled the surrounding skies.
Greece responded by sending its own military assets to the area to monitor Turkey's activities.
Erdogan appeared to suggest in a speech in the Turkish capital that the Oruc Reis had come under attack during its expedition and that Ankara had responded accordingly.
"We told them, don't you dare attack our Oruc Reis. You will pay a heavy price if you attack our Oruc Reis, we said. And they got their first answer today," Erdogan said.
He provided no details and immediately moved on to another topic in his wide-ranging address.
The Greek defence ministry denied any involvement.
"No incident happened," a Greek defence official told AFP.
- EU meeting -
Erdogan's cryptic comments followed his telephone talks on Thursday with German Chancellor Angela Merkel and EU chief Charles Michel.
Both Berlin and Brussels issued brief statements confirming that the Mediterranean tensions were covered but offering no specifics.
The Turkish presidency said Erdogan stressed the need for more diplomacy in both calls.
Erdogan told Merkel "he prefers disputes in the eastern Mediterranean are resolved within the framework of international law and on the basis of fairness and dialogue," the Turkish presidency said in a statement.
The presidency said Erdogan told the European Council president that Turkey favoured a solution "which will protect the rights of all countries and benefit everyone".
But Erdogan also reaffirmed to Michel "his commitment to defend Turkey's rights against attempts to disregard them," the Turkish presidency said.
The emerging crisis prompted France -- which has been at loggerhead with Turkey for months over Libya -- to announce it was "temporarily reinforcing" its military presence in the Mediterranean in support of Greece.
France has been critical of what it says are Turkey's violations of the sovereignty of Greece and Cyprus.
This uneasy relationship was complicated further when Paris accused Turkish ships of being "extremely aggressive" towards a French navy vessel in June.
Brussels will try to find a way to head off the rapid escalation during a meeting of EU foreign minister on Friday.
The EU has repeatedly urged Turkey to halt exploration work and sided with Greece's interpretation of maritime boundaries.
Erdogan has adamantly defended Turkey's right to search for oil and gas on its "continental shelf".
These tensions could linger as regional powers pour ever growing resources into disputed waters around Cyprus and other parts of the eastern Mediterranean and the Aegean Sea.
The Oruc Reis is scheduled to complete its mission on August 23.