ASHINGTON, Feb. 8 - Driven by new evidence that North Korea may have begun selling nuclear materials around the world, President Bush sent an emissary last week to see President Hu Jintao of China and urge him to intensify diplomatic pressure on the North to give up its weapons program, according to senior American and Asian officials.

The emissary, Michael Green, delivered a letter from Mr. Bush to Mr. Hu that, in the words of one American official, "was written to underscore the greatly heightened urgency" of the problem.

According to Asian officials, the Chinese promised to send a delegation to Pyongyang later this month, but also advised Mr. Bush against making public pronouncements about the North Korean situation, the way he regularly talked about the threat posed by Iraq in the year leading up to the March 2003 invasion. Mr. Bush has never publicly mentioned the new information about suspect North Korean nuclear sales, which was reported by The New York Times last week.

"The Chinese advised that we not demonstrate to the North how anxious everyone is about this," said one senior Asian diplomat who is deeply involved in the six-country negotiations over North Korea's nuclear program. "But the Chinese also seemed surprised by the quality of the scientific evidence," the diplomat added, that North Korea had produced several tons of a uranium compound that ended up in Libya. Until now, the Chinese, at least in public, had dismissed American charges that North Korea had a secret program to build weapons from uranium, based on technology it obtained from A. Q. Khan, the Pakistani nuclear scientist.

In response to inquiries last week, the White House confirmed that Mr. Green, who is the senior director for Asia at the National Security Council, had traveled to Asia with William Tobey, a National Security Council official who deals with the spread of nuclear weapons. White House officials said the two men were visiting Beijing, Tokyo and Seoul "to advance the goals of peace and stability within Asia and the world."

But they did not disclose the highly unusual meeting with Mr. Hu. Chinese leaders do not ordinarily see midlevel officials, but the two men arrived with American intelligence information about North Korea's two parallel nuclear weapons programs - one program that the North has boasted about, involving the manufacture of plutonium, and a second one that it has denied pursuing, involving uranium.

The meeting was intended to underscore the urgency of the new intelligence data, the first evidence that North Korea had gone beyond building new weapons for its own small nuclear arsenal, and may have provided aid to other nations by selling a partly processed form of uranium that can be enriched into bomb fuel. While there is some dispute about when the uranium was sent to Libya, there is some evidence that the transaction took place as recently as early 2003.

American and Asian intelligence officials say it is unclear whether North Korea knew that Libya was the ultimate destination for the chemical, called uranium hexafluoride. One senior official with access to the intelligence data said it was possible that the North Koreans only knew that it was transferring the fuel to members of Dr. Khan's network.

"We don't know how much they produced, or if it was shipped elsewhere," the official said. "It's one of the questions we have to get answered."

North Korea has not said whether it will rejoin the stalled negotiations, which also involve Russia, Japan and South Korea in addition to the United States and China. After a lengthy struggle within the Bush administration, the United States made a proposal in June that envisioned gradual economic aid and investment in North Korea in return for highly intrusive inspections and an agreement for complete dismantling of all of its nuclear facilities. The North Koreans have never responded to that proposal.

Mr. Bush has often repeated his determination to find a diplomatic solution, in part because his military options are unpalatable: Officials do not know the locations of some of North Korea's nuclear installations, apparently including the site of the plant that is producing the uranium hexafluoride. Nor do they know where North Korea is storing plutonium "cores," perhaps enough for six to eight weapons, that have probably been manufactured within the past two years, according to the C.I.A. Still, the evidence of sales of raw fuel has energized some hard-line officials in the administration, and renewed internal debate about taking actions to cut off North Korea's money and trade, and to destabilize the government of Kim Jong Il.

Mr. Bush's aides have determined that if the negotiations fail, they must end with China on the American side of the issue, not on the side of North Korea. "This is part of a strategy by the Bush administration to get China much more active on an issue that the administration has struggled with, and recognizes is now going on to the front burner," said Kurt Campbell, a former senior Pentagon official in the Clinton administration who worked on North Korean issues, and is now a senior vice president at the Center for Strategic and International Studies.

The central piece of evidence that Mr. Green and Mr. Tobey took to Asia was a scientific analysis of the uranium found in Libya, which has now been identified with near certainty as having been produced in North Korea, intelligence officials say. Adding to the evidence, they told Asian officials, were traces of plutonium on the outside of casks found in Libya, which they said appeared to match plutonium from the North's main nuclear site, at Yongbyon.

Two scientists who consult for federal intelligence agencies, but have not seen the evidence, said the mere presence of plutonium suggested a link to North Korea, because it leaves a distinctive "fingerprint." The United States, one of the scientists added, obtained a sample of North Korea's spent reactor fuel in 1992 from the International Atomic Energy Agency. Experts probably would have been able to compare the signature of the plutonium traces found in Libya with the sample in the possession of the United States.

Moreover, they noted, for more than a decade the North Koreans have reprocessed spent fuel from nuclear reactors to recover plutonium. Such reprocessing, they noted, also produces uranium as a waste product often contaminated with traces of plutonium.

Finally, the scientists said, the plutonium would contain mixtures of isotopes, or different forms of the element, that would reveal the type of reactor in which it was made.

David E. Sanger reported from Washington for this article and William J. Broad from New York.