Germany on Tuesday urged Turkey not to dispel the "climate of trust" needed to de-escalate tensions with Greece over Ankara's deployment of exploration vessels in the eastern Mediterranean.
The research ship Oruc Reis' presence "goes against what we (EU countries) agreed on, namely to try all diplomatic means to calm the intensity of tensions in a climate of trust," Foreign Minster Heiko Maas said after visiting Greek Prime Minister Kyriakos Mitsotakis in Athens.
"We've all been very surprised by what we've seen from the Turkish side in recent days," Maas said, speaking through an interpreter.
Ankara on Monday raised tensions with its neighbours after it said it would return the seismic research vessel, Oruc Reis, to the eastern Mediterranean, including off the coast of Greek islands.
The ship triggered diplomatic tensions and rival naval manoeuvres in August when it searched for oil and gas deposits, before Turkey pulled it back to shore last month.
"We had agreed with Turkey to restart exploratory talks," Mitsotakis said after his meeting with Maas.
"But instead of naming a date" to open negotiations, "we have the Oruc Reis on the Greek continental shelf. We can't talk under these conditions," he added.
Speaking earlier Tuesday in Cypriot capital Nicosia, Germany's Maas said that Turkey should make sure that "the window of dialogue that has just opened with Greece doesn't close because of unilateral measures."
The Greece-Turkey dispute will be on the agenda when EU leaders meet at a Brussels summit on Thursday and Friday.
"If Ankara continues its illegal actions, we will use all the instruments at our disposal" to respond, European Commission chief Ursula von der Leyen has said.