WASHINGTON (Reuters) - U.S. policy is to achieve a verifiable denuclearization of the Korean Peninsula, a State Department spokesman said on Tuesday, shortly after a senior U.S. official said trying to persuade Pyongyang to give up nuclear weapons was a lost cause. The comment by State Department spokesman John Kirby at a news briefing followed remarks by the Director of U.S. National Intelligence James Clapper at a Washington think tank that the U.S. policy of trying to persuade North Korea to denuclearize “is probably a lost cause.” 