The European Union's executive on Monday called on Turkey to "refrain from any actions that might damage good neighbourly relations" after Cyprus, a member of the bloc, accused the Turkish military of obstructing a ship exploring for gas.
Cyprus is one of several states, also including Israel and Lebanon, racing to tap gas deposits in the eastern Mediterranean.
Greek Cypriots run Cyprus's internationally recognised government while Turkish Cypriots run a breakaway state in the north recognised only by Ankara and say resources around the island belong to them too.
Cyprus on Sunday said the Turkish military had obstructed a vessel contracted by Italian oil company Eni which was approaching an area to explore for natural gas.
"Turkey needs to commit unequivocally to neighbourly relations and avoid any kind of source of friction, threat or action directed against a member state," a spokeswoman for the executive European Commission said.
"The EU also stresses the need to respect the sovereignty of member states over their territorial sea and airspace."
Greece accused Turkey of violating international law and said its "provocative conduct" was inconsistent with that of country seeking accession to the EU.
"We call on Turkey to desist from further unlawful actions and comply with its obligations stemming from international law," the foreign ministry in Athens said in a statement.
Relations between the EU and Turkey have become strained by disagreements over democracy and human rights, especially since a crackdown in the wake of a failed Turkish coup in 2016. However, European countries still rely on Turkey as a NATO ally and to slow an influx of Syrian refugees into the bloc.