The chief of Libya's eastern-based parliament has expressed his disagreement with an accord between Tripoli and Ankara establishing maritime boundaries.
Aguila Saleh made the comments on Thursday during a visit to Greece, which has strongly condemned the agreement signed late last month between neighbouring Turkey and Libya's internationally recognised government.
"We are here to stress that the deal is invalid. Those who signed it lack legality," Saleh, who was invited for talks in Athens, said through a translator.
An ally of renegade General Khalifa Haftar, a rival of the Tripoli-based Government of National Accord (GNA), Saleh said he would be in "close contact" with Greek officials to "neutralise" the agreement.
"We will resist anyone attempting to interfere in Libyan internal affairs," he said.
Infuriated by the accord, Greece accused the GNA of deception and expelled the Libyan ambassador to Athens. It also said it had lodged objections with the United Nations, saying the accord violated international law.
On Wednesday, the UN called on Greece and Turkey to maintain dialogue and there were no signs of a Security Council meeting on the issue.
Turkey says the accord aims to protect its rights and is in line with international law. President Recep Tayyip Erdogan said that it would allow Turkey and Libya to hold joint exploration operations in the region.
In Brussels for an EU summit on Thursday, Greek Prime Minister Kyriakos Mitsotakis called the deal "invalid" and claimed that "Europe is raising diplomatic walls against Turkish aggression".
Ankara has been one of the GNA's staunchest supporters, a relationship that has deepened in the face of an assault by Haftar to seize the Libyan capital since April.
Erdogan said this week that he was ready to send troops if they were requested by Tripoli.
The maritime deal is particularly important given the recent discovery of vast gas reserves that has triggered an exploration scramble between adjacent states and international oil companies.
Turkey already faces European Union sanctions over ships searching for oil and gas off Cyprus, which is not recognised by Turkey.
Cyprus has been practically divided since 1974, when Turkey militarily intervened on the island in response to a brief Greek-inspired coup. The Turkish Republic of Northern Cyprus, formed on the island's north in the following years after the intervention, is only recognised by Ankara.
Since 2014, Libya has had competing governments based in Tripoli and the east. Saleh's Tobruk-based House of Representatives was Libya's internationally recognised chamber at the time of a UN-backed deal in 2015 that led to the formation of the GNA, but the chamber has since fragmented.