By Sue-Lin Wong | PYONGYANG PYONGYANG **rth Korean state media warned on Tuesday of a nuclear attack on the United States at any sign of American aggression as a U.S. Navy strike group steamed towards the western Pacific.U.S. President Donald Trump, who has urged China to do more to rein in its impoverished ally and neighbour, said in a Tweet that **rth Korea was "looking for trouble" and the United States would "solve the problem" with or without Beijing's help.Tension has escalated sharply on the Korean peninsula amid concerns that reclusive **rth Korea may soon conduct a sixth nuclear test and after Washington said at the weekend it was diverting the aircraft carrier strike group Carl Vinson towards the Korean peninsula in a show of force.U.S