Turkey's president on Thursday threatened Greece against rising tensions in the eastern Mediterranean, after Turkish gas exploration vessel Oruc Reis resumed operations there earlier this week.

"We have told Greece if you attack our Oruc Reis, you will pay a heavy price. And they got their first answer today," Erdogan told fellow AKP party members in Ankara, without going into further detail.

Turkey on Monday sent the vessel to explore off the coast of the Greek island of Kastellorizo, accompanied by several Navy ships. Greece responded by sending its own military vessels to monitor the situation.

Erdogan's remarks come after unconfirmed reports in Greek media that a vessel from Greece hit a Turkish ship escorting the Oruc Reis. The Greek Defense Ministry denied attacking the Turkis ship, with a ministry spokesperson telling AFP: "No incident happened."

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Turkey and Greece — old rivals, new disputes

Greece and Turkey have been at loggerheads over energy resources since the discovery of hydrocarbon reserves in the eastern Mediterranean. Greece and the European Union claim Turkey is illegally drilling in the region, but Turkey claims the area is within its exclusive economic zone.

Earlier on Thursday, Erdogan held talks with German Chancellor Angela Merkel, saying "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.

Merkel brokered talks between Turkey and Greece in July in which Ankara agreed to temporarily pause drilling in the region. However, it resumed energy exploration Monday after Greece reached a deal with Egypt over rights to drill for oil and gas in the area.

EU foreign ministers are due to discuss the issue during a special video conference on Friday.

Meanwhile, France on Wednesday said it will boost its military presence in the eastern Mediterranean. A statement by President Emmanuel Macron's office said that France will "temporarily reinforce" the presence of its military to "monitor the situation in the region and mark its determination to uphold international law."

dr/dj  (AFP, dpa, AP)