Turkey has been condemned by the EU for its "unilateral actions and provocations" in the eastern Mediterranean by sending a natural gas exploration ship into contested waters.

Greece and Cyprus are in a stand-off over energy resources in the region with Turkey, which sent a research ship back to contested waters in defiance of international calls to withdraw.

The dispute was discussed at a Brussels summit of European leaders where they criticized the Turkey's actions.

The European Union "deplores renewed unilateral and provocative actions by Turkey in the eastern Mediterranean, including recent exploratory activities," the 27 leaders said in their summit communique.

They urged Turkey to reverse its recent activity and reiterated their "full solidarity" with EU members Greece and Cyprus.

"We agreed that the recent unilateral measures taken by Turkey, which are of course provocative, are now increasing tensions again instead of easing them," German Chancellor Angela Merkel said.

"I think this is very regrettable, but it is also not necessary. We should work on the positive aspects of our agenda."

EU leaders had a lengthy discussion over their relations with Ankara at a summit just two weeks ago, but Athens and Nicosia put grievances over Turkish energy exploration in the eastern Mediterranean back on the agenda.

After a similar row in August, Ankara has redeployed the research ship Oruc Reis to strategic waters between Cyprus and the Greek islands of Crete and Kastellorizo.

The U.S. and Germany, both NATO allies of Greece and Turkey, have labeled the gas exploration mission a "provocation" and urged Ankara to recall the ship.

But despite the leaders' strong rhetoric in their statement, EU Council President Charles Michel said there would be no change to the strategy agreed at the last summit.

Under this plan, the EU will closely monitor Turkey's actions in the eastern Mediterranean and decide on possible action at a summit in December.

The bloc has warned Turkey that all options are on the table, including sanctions. Since it has no armed forces, military action is not an option.

French President Emmanuel Macron, who has strongly supported Greece, even to the extent of holding joint war games in the Mediterranean as a show of strength, said Europe was ready to talk to Ankara.

But he warned "we will not concede anything to these provocations."