EU leaders on Friday condemned "unilateral actions and provocations" by Turkey in the eastern Mediterranean, where it is locked in a standoff over energy resources with Greece and Cyprus.
European leaders discussed the dispute at a summit in Brussels, after Turkey sent a research ship back to contested waters in defiance of international calls to withdraw.
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 of 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 United States and Germany, both NATO allies of Greece and Turkey, have labelled the gas exploration mission a "provocation" and urged Ankara to recall the ship.
On Friday, Secretary of State Mike Pompeo also criticized the self-styled state in northern Cyprus for reopening public access to Varosha, a seaside town that had been deserted since Turkish forces seized the north of the island in 1974.
Pompeo, in a telephone conversation with Cypriot Foreign Minister Nikos Christodoulides, "expressed deep concern and noted such a move was provocative and inconsistent" with UN Security Council resolutions and urged a reversal of the reopening, the State Department said.
Despite EU leaders' strong rhetoric in their statement, AFP quoted EU Council President Charles Michel as saying 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".