French sailors were trying to check a cargo ship on suspicion it was taking arms to Libya -- forbidden under a UN embargo.

Turkish frigates carried out radar targeting three times, suggesting a missile strike was imminent, the French defence ministry said.

"This is an extremely aggressive act that is unacceptable by an ally against a NATO ship," said a ministry official who did not want to be identified by name.

"We consider this an extremely grave matter... (and) we cannot accept that an ally behaves this way, that it does this against a NATO ship, under NATO command, carrying out a NATO mission," the official said.

French Defence Minister Florence Parly later brought the matter up during a videoconference with her NATO counterparts, her office said.

Tensions between France and Turkey, which is also a NATO member, have flared recently over their roles in the civil war in Libya.

Turkey supports the UN-recognised government in Tripoli, while France is suspected of favouring military strongman Khalifa Haftar, based in eastern Libya.

Turkey's support for the "offensive" of the UN-recognised Government of National Accord (GNA) runs directly against efforts to secure a ceasefire, a French foreign ministry spokesman said.

This week, Ankara condemned as "unacceptable" criticism from France over its shipments of weapons and fighters to the GNA, which Paris calls a "direct violation" of a United Nations embargo.

"These waves of boats between Turkey and Misrata, sometimes escorted by Turkish frigates, are not contributing to any de-escalation," the French ministry official said Wednesday.

Turkish Foreign Minister Mevlut Cavusoglu on Wednesday paid a surprise visit to Tripoli.

Oil-rich Libya has been torn by years of violence, drawing in tribal militias, jihadists and mercenaries since the 2011 toppling and killing of longtime dictator Moamer Kadhafi in a Western-backed uprising.

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