Heads of states of France, Greece, Cyprus, Malta, Italy, Spain and Portugal warned Turkey that "unless it ends its unilateral activities", the EU would "develop a list of further restrictive measures".
Leaders of seven southern European countries on Thursday urged Turkey to end "unilateral and illegal activities" in the eastern Mediterranean and resume dialogue to ease tensions in the region.
Heads of states and government of France, Greece, Cyprus, Malta, Italy, Spain and Portugal gathered in Corsica amid fears of open conflict as Turkey seeks to expand its energy resources and influence in the region.
In their final statement, leaders reaffirmed their "full support and solidarity with Cyprus and Greece" who they say are facing Turkey's "confrontational actions."
"We regret that Turkey has not responded to the repeated calls by the European Union to end its unilateral and illegal activities," they said.
Leaders warned that "in absence of progress in engaging Turkey into a dialogue and unless it ends its unilateral activities, the EU is ready to develop a list of further restrictive measures" at a summit later this month.
They also called on resuming German mediation in the dispute. Russia also offered this week to mediate.
'Respect international law'
Greece and Turkey have deployed naval and air force units to assert competing claims over energy exploration rights in the eastern Mediterranean. Turkish survey vessels and drill ships continue to prospect for gas in waters where Greece and Cyprus claim exclusive economic rights.
France is carrying out military patrols in the region in a show of support for Greece and Cyprus, and the EU is mulling new sanctions against Turkey.
Greek Prime Minister Kyriakos Mitsotakis said "if Turkey really wants a frank dialogue with Greece and Cyprus with the European Union, it must demonstrate this in practice: to immediately stop unilateral actions, to make convincing indications that it respect international law."
Turkey needs to "restrain its aggressive rhetoric" and "return to the table for exploratory talks with Greece," he added.
Turkish leaders have lashed out at France and the EU for siding with Greece and Cyprus in the dispute.
Earlier Thursday, Macron denounced what he called "unacceptable" provocations from Turkey.
"Turkey is no longer a partner in this region," Macron told reporters ahead of the summit. "We Europeans need to be clear and firm" with the government of Turkish President Recep Tayyip Erdogan about its "inadmissible behaviour," he said.
'A major destabilising factor'
Turkey's Foreign Ministry characterized Macron's statement as "arrogant" and in line with "old colonial reflexes." It accused the French president of stoking tensions and putting the "greater interests" of Europe at risk.
"It is not for Macron to determine the maritime jurisdiction of any country in the Mediterranean" or anywhere else, the Turkish ministry said in a statement.
Speaking Thursday to EU lawmakers, Greek European Affairs Minister Miltiadis Varvitsiotis appealed for support from European partners, saying the tensions over energy rights "constitute by themselves a grave threat to our common security architecture."
He said that Turkey is looking beyond Greece and is "a major destabilising factor in the wider area," citing Turkish government actions in Libya, Syria and beyond.
He said that Greece would not provoke conflict but wouldn't just sit back waiting for European help to arrive: "At the end of the day, we will defend ourselves, even alone."
'We are ready for dialogue'
Separately from the diplomatic discussions, Turkish and Greek military officials met Thursday at NATO headquarters, as part of ongoing meetings aimed at reducing the risk of armed conflict. Greece and Turkey both are NATO members.
The leaders also planned to discuss EU and NATO operations in the Mediterranean and their relation to Turkey during a dinner on Thursday evening.
The seven countries are aiming at coming up with a united southern European front before a full EU summit later this month focused on the bloc's strategy toward Turkey.
In a testy exchange with EU lawmakers, Turkish Foreign Minister Mevlut Cavusoglu urged the Europeans to play the role of an "honest broker" in the maritime border and energy dispute rather than take sides with member countries Greece and Cyprus.
"By acting as an international court, defending one side's claims over the issue, the EU has become a part of the problem unfortunately," Cavusoglu said by video link.
"We are ready for dialogue without any preconditions. If Greece insists on preconditions, we will also insist on our preconditions," Cavusoglu said, but added that "we are not for tension. We are not for escalation."
European Council President Charles Michel will travel to Greece, Cyprus and Malta next week for talks with leaders.