A senior U.S. official has identified North Korea, Iran, China, Russia and the Islamic State militant group as the biggest threats to the United States.Speaking at the Defense One Summit in Washington this week, U.S. Deputy Defense Secretary Robert Work said the global security situation has changed rapidly over the past five years.Work said threats coming from Russia and the Islamic State is growing in addition to Iran, China and North Korea, and he called for a 'grand strategy' to counter growing risks.