EU members will stand off over the thorny issue of sanctions against Turkey over the course of a two-day summit.
The 26-member bloc has reached an impasse over sanctions against Ankara due to Turkish naval actions in the eastern Mediterranean in recent weeks.
The stalemate has also affected Brussels’ ability to deal effectively with unrest in Belarus and has raised questions about the EU’s decision-making process on sanctions.
In his invitation to the special summit, EU council president Charles Michel said all options remained on the table with regard to Turkey but he highlighted that Brussels was seeking dialogue with Ankara.
As leaders meet on Thursday and Friday, it appears Cyprus in particular has lost the battle over sanctions in favour of negotiations between Brussels and Ankara.
“Our objective is to create a space for a constructive dialogue with Turkey to achieve stability and security in the whole region, and to ensure full respect for the sovereignty and sovereign rights of all EU member states,” Mr Michel said.
“This will only be possible if Turkey engages constructively. All options remain on the table to defend the legitimate interests of the EU and its member states.”
A dispute between Greece and Cyprus on one side and Turkey on the other over maritime borders and exploration rights in resource-rich waters in the eastern Mediterranean has boiled over this year.
Greece and Turkey, two Nato allies, have come close to direct confrontation after the latter sent research vessel Oruc Reis into Greek waters with half a dozen warships. Greece responded by mobilising the bulk of its fleet.
While the Oruc Reis has now withdrawn, easing tensions, the dispute over the waters continues.
Nicosia has a separate dispute with Ankara over exploration rights in its waters.
Last month, European foreign ministers failed to break the deadlock over sanctions on Belarus because of disagreements on sanctions against Turkey.
The Cypriot government refused to endorse sanctions against Belarusian officials because of the lack of action against Turkey. On Wednesday, Cyprus indicated it was prepared to stand its ground on the issue.
"To release the Belarus file we have to have an agreement on our proposals as well," a Cypriot diplomat said, adding that Cypriot President Nicos Anastasiades met Mr Michel, who will chair the two-day summit, on Wednesday to lay out Nicosia's position.
"I imagine there will be a long discussion in the European Council. I'm not excluding that something might come out of it but, as of now, I wouldn't put money on having a happy outcome."
Cypriot officials cited an August agreement among EU foreign ministers to approve sanctions against both Belarus and Turkey, arguing each is of equal importance.
The draft summit conclusions had no agreed line on Turkey.
Thursday evening’s dinner meetings will be dedicated to council discussions on Turkey. EU diplomats say a solution could involve a promise to Cyprus for tough sanctions on Turkey in the future.
“The idea is to threaten Turkey with retaliatory measures if it continues with drilling and other provocations in Cypriot and Greek waters,” a second senior EU diplomat said.
“This is meant to offer guarantees to Cyprus and convince Nicosia to lift its veto on Belarus sanctions.”
Ahead of the summit, Turkish President Tayyip Erdogan sent a letter to EU leaders - excluding Greece and Cyprus - saying Greece and Greek Cypriots had caused the regional tension, adding he hopes they will show an unbiased approach on Thursday.
"Our expectation from the EU is to remain objective, treat everyone equally and back dialogue and co-operation," Mr Erdogan said in the letter sent on Wednesday wrote.