Greece has complained to the UN in two letters against a maritime boundary deal between Turkey and Libya.
Athens has asked for the matter to be addressed by the UN Security Council, a government spokesman said on Tuesday.
Greek Foreign Minister Nikos Dendias convened a meeting in Athens to brief political party leaders on developments.
The deal, endorsed by Turkey's Parliament last week, has increased regional tension, particularly over rights for gas and oil exploration.
The agreement would give Turkey and Libya access to an economic zone across the Mediterranean despite the objections of Greece, Egypt and Cyprus, which lie between the two.
All three countries have condemned the deal as contrary to international law, and Greece expelled the Libyan ambassador last week over the issue.
Government spokesman Stelios Petsas said Greece sent one letter to UN Secretary General Antonio Guterres and one to the head of the UN Security Council on Monday night.
Mr Petsas said the letters noted the deal "was done in bad faith and violates the law of the sea, as the sea zones of Turkey and Libya are not neighbouring, nor is there a joint maritime border between the two countries".
The letters say the deal "does not take into account the Greek islands" and their right to a continental shelf and exclusive economic zone.
The agreement has not been ratified by Libya's Parliament, Mr Petsas said, rendering it "void and unable to affect Greek sovereign rights".
Greece and Turkey, although Nato allies, have tense relations and are divided by decades-old disputes, including territorial issues in the Aegean Sea, and have come to the brink of war three times since the 1970s, including once over drilling rights in the Aegean.
"Greece will do whatever is needed, and it knows what to do, in order to defend its sovereign rights," Mr Petsas said.
He said the letter to Mr Guterres asked for the deal not to be published by the organisation's division for ocean affairs and the law of the sea.
Mr Dendias raised the issue on Monday at a meeting of EU foreign ministers in Brussels, gathering clear statements of support from some of them.
"I want to tell Greek society that the country is dealing with the very serious problems with seriousness and confidence," he said after briefing party leaders.
EU foreign policy chief Josep Borrell said some of the bloc's members were concerned but the EU would study the deal further before deciding whether to take action.
"It's clear that it is problematic. It poses major concerns to certain member states, in particular Greece and Cyprus," Mr Borrell said.
"We are going to be studying this question very closely, being clear that any agreement must respect international law."
Turkish President Recep Tayyip Erdogan said on Tuesday that he was ready to send troops to Libya if requested by the internationally recognised government in Tripoli.
"If Libya makes such a request from us, we can send our personnel there, especially after striking the military security agreement," Mr Erdogan said in a televised appearance.
Turkey signed a military agreement last month with Libya's Government of National Accord, led by Prime Minister Fayez Al Sarraj.
