The EU says it could impose sanctions on Turkey over “provocations and pressures” in a row with Greece over energy resources and maritime borders.
European Commission President Ursula von der Leyen called on Ankara to “abstain from unilateral actions” in the eastern Mediterranean.
She spoke early on Friday during a meeting of EU leaders in Brussels.
Earlier, Turkey and Greece set up a military hotline to try to reduce the risk of clashes in the region.
Tensions rose earlier this year when Turkey sent a ship into a disputed area to search for potentially rich oil and gas deposits.