Greece said on Friday that Libya's ambassador to the country was to be expelled following a controversial maritime accord reached by Tripoli and Ankara.
The deal maps out a sea boundary between Turkey and Libya close to the Greek island of Crete.
Greece called the accord a "blatant violation of international law" whereas Ankara claims it is "protecting its rights."
Libya's Foreign Minister Mohamed Siyala said Athens' decision was "unacceptable," while Turkey also criticised the move.
"Expelling an ambassador just because of the (agreement) that we signed is not a mature behaviour in diplomacy. This is outrageous," Turkey's Foreign Minister Mevlut Cavusoglu told reporters in televised comments during a visit to Rome.
'Violation of international law'
"This is a legally invalid document," Greek Prime Minister Kyriakos Mitsotakis told parliament.
"Not only is it geographically and historically invalid - wiping Greek islands off the map - but because it led Turkey to an unprecedented diplomatic isolation," he said.
"It's just a piece of paper nobody recognises."
According to Turkish news agency Anadolu, "the document reaffirmed the rights of Turkey and the Turkish Republic of Northern Cyprus to resources in the Eastern Mediterranean in the face of unilateral drilling by the Greek Cypriot administration."
Libyan Ambassador Mohamed Younis AB Menfi has 72 hours to leave the country, Greek Foreign Minister Nikos Dendias told a news briefing.
The move did not mean Greece was severing diplomatic relations with Libya, Dendias added.
The expulsion is the latest twist in a saga of Mediterranean states jostling to claim yet-untapped oil and gas in the region.
It comes as Turkey's parliament and Libya's presidential council ratified Thursday their memorandum of understanding on maritime zones.
Greece and Turkey are at odds over various decades-old issues ranging from mineral rights in the Aegean Sea to ethnically-split Cyprus.
Tensions are already running high between Athens and Ankara because of Turkish drilling in the eastern Mediterranean off the coast of Cyprus, and the European Union has prepared sanctions against Turkey in response.
Concerns across the region
Libya's neighbour Egypt dismissed the deal as "illegal" as did Cyprus, while Greece has said any such accord would be geographically absurd because it ignored the presence of the Greek island of Crete between the coasts of Turkey and Libya.
But Turkish Foreign Ministry spokesman Hami Aksoy said on Sunday that the agreement was "in accordance with the court decisions that create the international jurisprudence and international law including the relevant articles of the United Nations Convention on the Law of the Sea."
On Thursday, Cyprus said it was petitioning the International Court of Justice to safeguard its offshore rights.
Greek Prime Minister Kyriakos Mitsotakis met Turkish President Recep Tayyip Erdogan in Britain on Wednesday on the sidelines of a NATO summit.
"I raised all issues relating to the latest Turkish actions," Mitsotakis said in a statement. "The disagreements of both sides were recorded. The two sides, however, agreed to continue discussions on confidence-building measures."
"I want to reassure the Greek people that difficulties with Turkey existed, exist and will exist. But I assess that, provided the two sides show goodwill, these will be overcome," Mitsotakis said.