Even as the White House presses China to find a solution to the nuclear issue, Chinese officials say, it has hurled insults at North Korea and given its leaders excuses to stay away from the bargaining table.

"It is true that we do not yet have tangible achievements" in ending North Korea's nuclear weapons program, Mr. Yang said in an interview. "But a basic reason for the unsuccessful effort lies in the lack of cooperation from the U.S. side."

Mr. Yang said that when President Bush referred to the North Korea leader, Kim Jong Il, as a "tyrant" in late April, Mr. Bush "destroyed the atmosphere" for negotiations, undoing weeks of efforts to persuade North Korea that the United States would bargain in good faith.

China, which has used its diplomatic clout to try to broker a peaceful solution to the nuclear crisis, has struggled to restart six-nation negotiations, which stalled nearly a year ago.

Mr. Yang said formally on Thursday what diplomats here had been whispering for months: personal attacks against Mr. Kim by Mr. Bush, Secretary of State Condoleezza Rice and other top administration officials had caused a "loss of face" for North Korean officials and created big obstacles to reaching any negotiated solution.

He urged the Bush administration to find some "informal channel" to talk with North Korean diplomats, perhaps over coffee or a meal, to build confidence. American officials have resisted any direct contact with North Koreans outside the six-nation talks. Mr. Yang said that without some new gestures the obstacles to resuming negotiations could prove insurmountable.

"I know the U.S. is reluctant to have even informal contacts" with North Korea, he said. "But as the world's superpower, I would hope it can show more flexibility and sincerity to make a resumption of talks possible."

The Beijing government is determined to head off a looming confrontation between the United States and North Korea, which it fears could prompt a regional nuclear arms race and shatter the stability that has underpinned China's own economic rise.

But the prospects for a negotiated solution have diminished after the recriminations between the United States and North Korea and warnings by American officials that North Korea has accelerated its development of nuclear bombs and may be preparing to test a nuclear device.

Mr. Yang said China would be "very concerned" about a nuclear test. But he said he doubted North Korea would take that step now, adding that China had made it "very, very clear" to North Korea that a test or any other provocative display of its nuclear capability would have serious consequences.

North Korea "understands the consequences very clearly," Mr. Yang said. "I do not think we should reach the conclusion that there will be a test."

Some American and Chinese analysts have speculated that North Korea may have made preparations for a test in full view of American spy satellites to create a sense of urgency about its nuclear program and lay the groundwork for demanding greater concessions if negotiations resume. But others say they believe North Korea is determined to become a full-fledged nuclear power and is prepared to weather penalties that may be imposed as it pursues that goal.

The United States and China worked closely together to organize multiple rounds of talks with North Korea that also included South Korea, Japan and Russia. Not since the two countries coordinated strategies against the former Soviet Union in the 1980's have they cooperated on a diplomatic project for such an extended period.

But tensions have risen as North Korea has appeared to be continuing to develop its nuclear arsenal and has resisted returning to the talks. Bush administration officials contend that China must begin using more economic and political leverage to pressure North Korea. China has rejected "strong-arm tactics" and suggested, usually in private, that the United States stop demonizing North Korea.

Mr. Yang expressed some puzzlement as to why the United States had pushed China to cut off oil or fuel supplies to North Korea - part of its lifeline of support for the government, which is in need of money - at the same time that it professed to want to resume negotiations.

"If you look at history you cannot find many successful cases in which sanctions achieved a successful result," he said.

Mr. Yang disputed an account of a meeting he held with Assistant Secretary of State Christopher Hill that was carried last week in The Washington Post. In that account, Mr. Yang was quoted as rejecting American demands to cut off North Korea's fuel supplies, but as indicating that China might withhold food aid as a way of forcing North Korea to resume talks.

Mr. Yang said Thursday that he did not discuss those options with Mr. Hill. He said he did not see the need for any penalties, involving food, oil shipments or other measures, as long as the talks moved ahead.