By David Brunnstrom and Simon Lewis WASHINGTON (Reuters) -North Korea could conduct a seventh nuclear test at “any time” and has shown no interest in returning to negotiations, the U.S. Special Representative for North Korea said on Tuesday. North Korea has test-launched an unprecedented number of ballistic missiles this year and its officials have also used rhetoric that could suggest plans for the use of tactical nuclear weapons, Sung Kim told reporters. Kim reiterated U.S. assessments that North Korea was preparing to conduct a seventh nuclear test. Asked when that might happen, he said: “I…