Donald Trump’s defence secretary has warned North Korea that it will face an “effective and overwhelming” response if it choses to use a nuclear weapon. <br /><br /> Speaking at the end of a two-day visit to South Korea, James Mattis insisted Washington was steadfast in its support for Seoul and would step in should the North launch an attack. <br /><br /> The US defence chiefs remarks come amid concern that the regime of Kim Jong-un is preparing to test-fire a new ballistic missile. <br /><br /> Last year, Pyongyang conducted more than 20 missile and two nuclear tests, in defiance of U.N. resolutions and sanctions.<br />
