Pentagon chief Jim Mattis has tried to clear up doubts about the US administration’s North Korea strategy, backing Secretary of State Rex Tillerson’s effort to find a diplomatic solution to the nuclear stand-off. Defence Secretary Mattis was speaking two days after US President Donald Trump appeared to undermine his top diplomat by saying Tillerson was “wasting his time” by maintaining contacts with Kim Jong-un’s regime. State Department officials insist Trump was not criticising Tillerson, but pressuring North to agree to discuss its disarmament while a diplomatic option remains on the table. Jim Mattis said: “The Defence Department supports fully Secretary Tillerson’s efforts to find a diplomatic solution but remains focused on defence of the United States and our allies”. Tillerson has explained the strategy as one of using United Nations and US sanctions and diplomatic pressure to convince Kim of his isolation and force him to negotiate nuclear disarmament. Trump wrote: “I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man”.