Yemen's Huthis fired three missiles at two merchant ships in the Red Sea on Wednesday in their latest attack in the commercially vital waterway, the White House and the Iran-backed rebels said.It came after the Huthis vowed to keep up their attacks despite repeated US and British strikes against them.One missile missed its target and a US Navy destroyer shot down the other two, National Security Council spokesman John Kirby said.The continuing Huthi action "means we're obviously still going to have to do what we have to do to protect that shipping," he added.