Contributed by: Don Winner(AP) NAIROBI, Kenya -- Somali pirates hijacked a cargo vessel Monday after firing rocket propelled grenades and small arms at the ship, the European Union Naval Force said Monday. The pirates seized the MV Orna on Monday about 400 miles (640 kilometers) northeast of the island nation of the Seychelles, said Wing Cmdr. Paddy O'Kennedy. The ship was not registered with maritime authorities so the number and nationalities of crew are not known. O'Kennedy said at least four pirates boarded the vessel and the crew are cooperating and have not reported any damage. The MV Orna is a Panama-flagged, bulk cargo vessel owned by a company in the United Arab Emirates. Before the hijacking of the MV Orna, the pirates were holding 23 ships and 555 hostages, according to the EU Naval Force.