WASHINGTON (Reuters) -The United States on Tuesday said it was not involved in an Israeli airstrike on Iran's embassy compound in the Syrian capital of Damascus that killed two Iranian generals and five military advisers. White House national security spokesman John Kirby dismissed as "nonsense" Iranian charges of U.S. responsibility for Monday's bombing, and he warned that Washington would respond to any retaliatory attacks. The Damascus strike threatened to escalate the years-long confrontatio