China told Asian diplomats last week it is not convinced of U.S. claims that North Korea has a clandestine program to enrich uranium for use in nuclear weapons, according to U.S. officials who have been briefed on the discussions. The previously unreported conversation — raising doubts about the central element in the Bush administration’s case against Pyongyang — underscores how Chinese and U.S. aims appear to be diverging in the diplomatic effort to restrain North Korea’s nuclear ambitions. China has taken the lead in organizing another round of six-nation talks, but the effort has bogged down over disputes among the parties about the scope and content of the negotiations. North Korea yesterday announced what it called a “bold concession” of offering to freeze both its nuclear weapons production and its nuclear power facility as “first-phase measures” of a package deal that would call for the United States to lift sanctions and provide energy aid. Secretary of State Colin L. Powell said the statement was “a positive step” that could lead to a rapid resumption of talks. Full Story