The Flora 1 was boarded on Friday after officials alleged there were issues with its registration status and that the vessel was on an EU sanctions list.

But the coast guard said late on Saturday that Cameroon had confirmed the vessel's registration and no environmental offence could be proved, adding: "There are therefore no further grounds for continuing the investigation."

European countries have ramped up action against a so-called "shadow fleet" of vessels that transport oil from Russia in an effort to get around EU and other international sanctions.

According to the ship tracking site marinetraffic.com, Flora 1 departed the Russian oil port Primorsk on March 31 heading for Santos in Brazil, flying the flag of Sierra Leone.