"North Korea is preparing for a third atomic test that may come in May or June, South Korean broadcaster YTN reported on Tuesday, an act that could further isolate Pyongyang and complicate already troubled nuclear diplomacy. South Korean Foreign Minister Yu Myung-hwan dismissed the report, saying Seoul had seen no evidence, and the U.S. State Department also voiced doubts about its accuracy. ""If North Korea was making such preparations, there would be related circumstances that can be detected ... there is no intelligence on such circumstances,"" Yu told a news briefing. ""We are skeptical of that report,"" State Department spokesman P.J. Crowley told reporters in Washington. YTN quoted an unnamed diplomat as saying that preparations began in February and involve a level of technical proficiency that is significantly upgraded from the first two tests, considered partial successes at best."