(AP) -- Spain formally laid claim Thursday to a shipwreck that yielded a $500 million treasure, saying it has proof the vessel was Spanish.

Officials demanded the return of the booty recovered last year by a U.S. deep-sea exploration firm, saying the 19th-century shipwreck at the heart of the dispute is the Nuestra Senora de las Mercedes - a Spanish warship sunk by the British navy southwest of Portugal in 1804 with more than 200 people on board.

Tampa, Fla.-based Odyssey Marine Exploration on Thursday disputed Spain's claim. In May 2007, Odyssey announced that it had discovered a wreck in the Atlantic - and its cargo of 500,000 silver coins and other artifacts worth an estimated $500 million.