French President Emmanuel Macron warned Thursday as he arrived at an EU summit that Europe's solidarity with member states Greece and Cyprus in their stand-off with Turkey must be "non-negotiable".

European powers are trying to defuse a dispute over maritime borders and energy fields in the eastern Mediterranean, but Greece and particularly Cyprus are pushing for a tough line on Ankara.

Cyprus has been blocking efforts by its EU partners to impose sanctions on actors in the Belarus political crisis, demanding that Brussels first take a stand on Turkish gas drilling in its waters.

"This solidarity is non-negotiable," said Macron, who regularly crosses diplomatic swords with Turkey's President Recep Tayyip Erdogan and has deployed warplanes to back up Greece.

"When a member state of the European Union is attacked, threatened, when its territorial waters are not respected, it is the duty of Europeans to show solidarity and we will reiterate our support for Greece and Cyprus, but we must also find ways to re-engage in a demanding dialogue with Turkey. 

"For me, these two cases are emblematic of what a re-thought, rigorous but also realistic neighbourhood policy is, that the European Union must have, which never compromises on issues of sovereignty, which never compromises on issues of values and law, but which seeks, with pragmatism and determination, to engage in a constructive dialogue."