US Defence Secretary Jim Mattis has said that North Korea risks annihilation if it starts a war. He told Pyongyang yesterday that it must end its pursuit of nuclear weaponry. <br/><br/>Mattis said North Korea should avoid actions that could usher in the end of its regime and the destruction of its people. <br/><br/>The Pentagon chief also said Pyongyang would be "grossly overmatched" in a war against the US and its allies. His stark warning comes a day after President Donald Trump threatened North Korea with fire and fury.<br/><br/>Meanwhile, the US Secretary of State Rex Tillerson assured Americans that there was no imminent threat from North Korea.