China publicly questioned the US allegations in the New York Times this month, prompting Washington to respond that it was "puzzled".

While North Korea acknowledges having a plutonium program, it denies it is enriching uranium to make nuclear fuel.

The United States is insisting that it completely dismantle both programs before receiving aid and security guarantees, a demand which has stalled two previous rounds of six-party talks that also involve Japan, Russia and South Korea.

"We don't know whether it exists. So far the United States has not presented convincing evidence of this program," deputy foreign minister Zhou Wenzhong said in the interview.

Washington has long said that it learned "conclusively" in the summer of 2002 that the North Koreans were pursuing a covert nuclear program based on uranium enrichment.

According to the paper, Zhou said the Bush administration should stop making charges about the program unless it could offer more conclusive evidence that it exists.

Up until now, Washington has been keen to let Beijing pull strings behind the scenes and pressure the regime of North Korean leader Kim Jong-Il while hosting the six-party talks.

But this could be backfiring, with China growing impatient with the US' uncompromising position.

"It's possible that the US recently shared with the Chinese the intelligence on which the uranium allegations are based, and the Chinese were unconvinced," said Timothy Savage, a visiting fellow at the Seoul-based Institute of Far Eastern Studies who specialises in Korean affairs.

He said the timing of the remarks was interesting, shortly before the third round of six-party talks that start in Beijing on Wednesday.

"As for why Zhou made the comments, there are a couple of possibilities," he said.

"One is that China really does believe that it is the US that is blocking progress in the talks and is trying to put pressure on them, especially as the Japanese now seem to be moving in the direction of engagement.

"Another, it may simply be that China is publicly moving closer to the North Korean position in order to make the North Koreans more comfortable about coming to the table, but that they'll continue to pressure them behind the scenes to give up the nuke program."

For China, progress in the talks, or at least preventing them from collapsing, is a matter of saving face.

Beijing launched its most significant diplomatic offensive in years to bring together Pyongyang and Washington last August for the first round of six-nation talks and has been working frantically ever since to prevent the dialogue from dying.

Ralph Cossa, president of the Pacific Forum CSIS, a Hawaii-based think-tank, said that Zhou's comments were likely not a fundamental change in the Chinese position, but rather a way to let Washington know it was tired of its foot-dragging.

"I would not call it a changed position but it is clear that China (and South Korea) would like the US to be more flexible and forthcoming on what it is prepared to offer North Korea if it behaves and comes clean," he said.

"China is certainly pressuring Washington to be more flexible and, more importantly, is demonstrating to Seoul that it is trying to be helpful but that Washington is the problem."

China is seen as one of the few states with any influence in Pyongyang courtesy of aid and oil, and is a long-time ally, fighting on North Korea's side against US and South Korean forces in the 1950-53 Korean War.

Marcus Norland, a senior fellow at the Washington-based Institute for Internal Economics, said it was possible the US could table better evidence this week to keep China onside.

"The US is likely to present more evidence regarding the DPRK's uranium program in an effort to keep China and others onside, but may not be more forthcoming about what sorts of carrots it is willing to offer."

WAR.WIRE