French President Emmanuel Macron has accused his Turkish counterpart Recep Tayyip Erdogan of breaking his promise to stay out of Libya.
Mr Macron said Turkish warships accompanied by Syrian mercenaries had been spotted arriving in Libyan.
Mr Erdogan has not responded to the allegations.
Earlier this month, world leaders pledged not to interfere in Libya's civil conflict and vowed to uphold a UN arms embargo.
Turkey supports the UN-backed Government of National Accord (GNA) in Tripoli while France has nurtured ties with the government's rival, the rebel General Khalifa Haftar.
Since April 2019, the GNA has fought back against an offensive launched by fighters loyal to Gen Haftar.
Mr Macron said the presence of Turkish warships was "a clear violation" of what President Erdogan had pledged in Berlin on 19 January during a conference on Libya with other world leaders. Mr Macron called Turkey's actions "detrimental to the security of all Europeans and Sahelians".
The Sahel region, a semi-arid stretch of land just south of the Sahara Desert, has been a frontline in the war against Islamist militancy for almost a decade. There are fears that arms from Libya could flood into the Sahel region, where 4,500 French soldiers and more than 14,000 UN peacekeepers are based.
What happened in Berlin?
Leaders from the EU, Russia and Turkey were among those who committed to end foreign intervention in Libya's war, and to uphold a UN arms embargo.
After the conference, German leader Angela Merkel stressed there was no military way to end the conflict, "only a political solution".
UN Secretary-General António Guterres, meanwhile, said all the major powers present shared "a strong commitment to stop" any further escalation in the region.
He added, however, that he was "very worried" about reports that forces loyal to Gen Haftar had closed several key ports and a major oil pipeline in the country.
Russian Foreign Minister Sergei Lavrov also expressed concerns. He said afterwards that the major powers "have not yet succeeded in launching a serious and stable dialogue" between the warring parties.
What is happening in Libya?
Libya has been torn by conflict since the 2011 uprising which ousted long-time strongman Muammar Gaddafi.
Gen Haftar's Libyan National Army (LNA) controls much of eastern Libya, and last April he launched an offensive against the GNA based in the capital, Tripoli.
His forces have so far been unable to take the city, but earlier this month the LNA captured the country's third-biggest city, Sirte.
According to the UN, the fighting has killed hundreds of people and displaced thousands more from their homes.
A truce was announced earlier this month between Gen Haftar and the GNA, led by Prime Minister Fayez al-Serraj.
But both sides blame each other for reported breaches of the agreement, and attempts to broker a lasting ceasefire broke down last week at a summit in Moscow.
What about the role of foreign powers?
The role of foreign states in the conflict has come into focus in recent months, with Turkey passing a controversial law to deploy troops to help GNA forces in Tripoli.
Gen Haftar's LNA has the backing of Russia, Egypt, Saudi Arabia, the United Arab Emirates (UAE) and Jordan.
Recently, UN Special Envoy to Libya Ghassan Salamé told the BBC's Radio 4 Today Programme that a political solution to the conflict was best for all parties involved because Libya - with its vast geography, strong local identities, heavily armed population and weakened government infrastructure - was a difficult country for one group to control.