The European Union (EU) may cancel a planned summit with Turkey next month because of tensions between Turkey and Greek Cyprus over energy exploration in the Mediterranean, Bloomberg reported on Friday.

The threat comes after Turkish naval vessels prevented drilling by Italy-based Eni SpA in waters that are part of Cyprus’ exclusive economic zone earlier this month.

“These actions contradict Turkey’s commitment to good neighbourly relations,” EU President Donald Tusk told reporters on Friday after an EU summit in Brussels.

Relations between Turkey and the EU have been going downhill since a failed coup attempt in Turkey in July 2016 that was followed by a Turkish government crackdown on political opponents. The acrimony has resulted in the effective freezing of negotiations on a bid for EU membership by Turkey.

“The time has come to send a very clear message to Turkey,” Greek Prime Minister Alexis Tsipras told reporters in Brussels. “EU-Turkey relations cannot progress with constant violations of sovereign rights of EU member states.”

The EU’s stance on Turkey is complicated by its dependence on Turkish assistance in preventing a renewed flood of Middle Eastern refugees entering the EU via Greece and Turkey's key role in fighting terrorism.

The planned summit, to be held in Varna, was intended to improve ties between Turkey and the EU.