China has cast doubt on U.S. assertions that North Korea has a uranium-based nuclear weapons development program, stirring up tensions with Washington on a delicate subject ahead of a new round of six-nation nuclear talks.

China's deputy foreign minister, Zhou Wenzhong, said in an interview with The New York Times that the United States has yet to persuade China that North Korea has a uranium program in addition to a plutonium program to develop fuel for nuclear bombs.

Zhou said Beijing doesn't know whether the alleged uranium program exists as claimed by the U.S., and that so far, the U.S. has not presented convincing evidence of its existence.

In Washington, State Department spokesman Richard Boucher said that the evidence presented to China was clear and characterized Zhou's comments as "somewhat puzzling."

The exchange of comments comes as State Secretary Colin Powell and his Chinese counterpart, Li Zhaoxing, get ready to meet in Washington Friday for talks on North Korea's nuclear issue.