DUBAI, United Arab Emirates—The U.S. Navy announced Sunday it seized an arms shipment of thousands of assault weapons, machines guns and sniper rifles hidden aboard a ship in the Arabian Sea, apparently bound for Yemen to support the country’s Houthi rebels. An American defense official told The Associated Press that the Navy’s initial investigation found the vessel came from Iran, again tying the Islamic Republic to arming the Houthis despite a United Nations arms embargo. Iran’s mission to the U.N. did not immediately respond to a request for comment, though Tehran has denied in the past giving the rebels weapons. The seizure, one of several amid the yearslong war in Yemen, comes as the United States and others try to end a conflict that spawned one of the world’s worst humanitarian disasters. The arms shipment, described as sizeable, shows that the war may still have far to run. The guided-missile …