Turkey and Greece have set up a military hotline to try to reduce the risk of clashes in the Mediterranean, where the two are locked in a row over energy resources and maritime borders.
The move was announced by the Nato military bloc, of which both countries are members.
Tensions rose this year when Turkey sent a research ship to a disputed area.
It comes as EU leaders met to discuss the bloc's thorny ties with Turkey.
Turkey has been a long-term candidate for membership of the European Union but efforts have stalled, with EU leaders criticising Turkey's record on human rights and the rule of law, in particular in the wake of the 2016 failed military coup.
But Turkey remains an important partner for the EU. Turkey hosts millions of migrants and struck a deal with the EU that limited the numbers arriving in Greece.
The announcement of a hotline followed talks between Turkey and Greece at the Nato headquarters in Brussels.
"I welcome the establishment of a military de-confliction mechanism, achieved through the constructive engagement of Greece and Turkey, both valued Nato allies," said Nato Secretary-General Jens Stoltenberg.
"This safety mechanism can help to create the space for diplomatic efforts to address the underlying dispute and we stand ready to develop it further."
Such mechanisms enable direct communication between two sides - Russia and the US set one up during the Cold War and it has been in operation ever since.
In August two Turkish and Greek war ships collided in the Eastern Mediterranean. Since then tensions have eased somewhat, with the Turkish research vessel leaving the area last month and both sides saying they were prepared to resume talks.
News of the hotline emerged as EU leaders arrived elsewhere in Brussels for a summit. The bloc has backed its members Cyprus and Greece against Turkey.
Greece's Prime Minister Kyriakos Mitsotakis said Turkish "provocations" had to stop.
"One thing is certain: Turkish provocation, whether manifested through unilateral actions or through extreme rhetoric, can no longer be tolerated," he said.
French President Emmanuel Macron said support for Greece and Cyprus - which also has claims on Mediterranean resources - was "non-negotiable", while Austrian Chancellor Sebastian Kurz has called for sanctions against Turkey.
Turkey's President Recep Tayyip Erdogan blames Greece and Cyprus for the tensions but in a letter to EU leaders said he wanted dialogue.
The EU is split on how to tackle Turkey, and divisions spilled into another key item on the agenda - Belarus, with the bloc refusing to recognise Alexander Lukashenko as the president following a disputed election.
Cyprus has been blocking EU attempts to impose sanctions on Belarus, wanting the EU to first impose them on Turkey.
All 27 members of the EU reject the results of the election in Belarus but Cyprus would need to drop its veto for sanctions to go ahead,
The EU's foreign policy chief, Josep Borrell, last month warned "our credibility is at stake" if an agreement could not be reached.
Talks continue for a second day on Friday, with officials still drafting a summit declaration.