Both France and Vanuatu claim the two uninhabited islands whose ownership has been the subject of negotiations between the two countries' governments.
Last week, Vanuatu summoned French diplomats after reports that the French navy had visited the island in January and painted a French flag on a rock.
Ten years ago, an FLNKS leader Victor Tutugoro signed a document with the Vanuatu Prime Minister Edward Natapei stating that the islands belonged to Vanuatu.
The move drew a rebuke from the French foreign minister who said it was difficult for New Caledonia to act as if it was independent when it was not.
France annexed the islands east of New Caledonia and south-east of Vanuatu in 1976 - four years before Vanuatu became independent of both Britain and France.
The maritime zone attached to the islands is bigger than the land area of New Zealand.