WASHINGTON, Feb. 1 - Scientific tests have led American intelligence agencies and government scientists to conclude with near certainty that North Korea sold processed uranium to Libya, bolstering earlier indications that the reclusive state exported sensitive fuel for atomic weapons, according to officials with access to the intelligence.
The determination, which has circulated among senior government officials in recent weeks, has touched off a hunt to determine if North Korea has also sold uranium to other countries, including Iran and Syria. So far, there is no evidence that such additional transactions took place.
Nonetheless, the conclusion about Libya, which is contained in a classified briefing that has been described to The New York Times, could alter Washington's debate about the assessment of the North Korean nuclear threat. In the past, some administration officials have argued that there is time to find a diplomatic solution because there was no evidence that the government of Kim Jung Il was spreading its atomic technology abroad.
Nine months ago, international inspectors came up with the first evidence that the North may have provided Libya with nearly two tons of uranium hexaflouride, the material that can be fed into nuclear centrifuges and enriched into bomb fuel. Libya surrendered its huge cask of the highly toxic material to the United States when it dismantled its nuclear program last year.
Now, intelligence officials say, extensive testing conducted at the Oak Ridge National Laboratory in Tennessee over the last several months has concluded that the material did not originate in Pakistan or other suspect countries, and one official said that "with a certainty of 90 percent or better, this stuff's from North Korea."
It is unclear if there are any dissenting views in the government, though some outside experts have accused the administration of overstating intelligence on North Korea. Officials cautioned that the analysis of the uranium had been hampered by the fact that the United States has no sample of known North Korean uranium for comparison with the Libya material. The study was done by eliminating other possible sources of uranium, a result that is less certain than the nuclear equivalent of matching DNA samples.
One recently retired Pentagon official who has long experience dealing with North Korea said the new finding was "huge, because it changes the whole equation with the North."
"It suggests we don't have time to sit around and wait for the outcome of negotiations," he said. "It's a scary conclusion because you don't know who else they may have sold to."
President Bush is expected to mention North Korea in his State of the Union address on Wednesday night. In that speech three years ago, he identified the country as part of an "Axis of Evil," along with Iran and Iraq. Two weeks ago Condoleezza Rice, in her confirmation hearings for secretary of state, included North Korea in a list of six "outposts of tyranny," but a senior administration official said Mr. Bush was not planning to use that phrase in his speech.
On Tuesday, in an interview with Reuters and Agence France-Presse, Ms. Rice said of North Korea, "We made a very good proposal at the last round of six-party talks and it's on the table for the taking."
"The idea that somehow the United States is hostilely going to attack North Korea couldn't be more far fetched," she said. A spokesman for the National Security Council, Frederick Jones, declined to comment Tuesday night on the report of the new North Korea findings, citing "intelligence concerns."
Questions of how to deal with North Korea - through engagement and dialogue or through sanctions and pressure to crack its government - have divided the Bush administration since its first days. Vice President Dick Cheney has led the hawkish faction, declaring that "time is not on our side." While some of the officials interviewed about the most recent North Korean evidence have been involved in that policy debate, others have not been, and have either examined the scientific evidence or received intelligence briefings about its conclusions.
Some findings have been shared with American allies and with the Chinese, who have long been dubious that North Korea has an active uranium program under way. There is also some skepticism in the United States: Selig Harrison, a North Korea scholar at the Woodrow Wilson International Center in Washington, has questioned the evidence that the North is secretly pursuing uranium weapons to complement the small arsenal it is believed to have produced out of plutonium. He wrote in the journal Foreign Affairs recently that the Bush administration has relied "on sketchy data" and "a worst-case assessment" of the North's capabilities.
The government's most recent intelligence reports, however, strongly suggest North Korea has begun turning raw uranium, which the country mines, into uranium hexaflouride, a modestly complex process.
"This pushes along our understanding of the North Korean program," said Leonard S. Specter, the deputy director of the Center for Nonproliferation Studies at the Monterey Institute in California. "It means the North Koreans have built a facility to process uranium. And it raises the disturbing prospect that they've now made enough of it to feel comfortable selling some."
Nuclear intelligence experts said the new clues that implicate North Korea as Libya's supplier involve the fingerprints of uranium isotopes, or different forms of the element. Federal analysts, they said, took samples of the Libyan uranium and compared its isotope fingerprint with those of uranium samples from other countries and, by process of elimination, concluded that the uranium had come from North Korea.
Uranium has three main isotopes. The most prevalent is U-238, which accounts for a vast majority of natural uranium. U-235 is rare, but it is prized because it easily splits to produce the bursts of atomic energy that power reactors and nuclear warheads.
To trace the Libyan uranium, the government sleuths focused on an even rarer isotope, U-234. They did so because it turns out that concentrations of that isotope vary widely among uranium deposits and mines around the world.
"The science is pretty clear," said a senior federal intelligence official knowledgeable of the secret North Korea finding.
The U-234 content "fluctuates over a wide range," the Russian scientists reported.
A nuclear scientist who consults for federal intelligence agencies but was unaware of the North Korean finding said analysts could use such U-234 information to track the origin of a uranium sample, much as detectives match fingerprints from a crime scene to archives.
He said the analysts could examine the U-234 concentrations in the Libyan sample and compare it with samples from deposits from around the world. Since Western intelligence agencies have no known samples of North Korean uranium, he added, the analysis would proceed by the process of elimination.
Therefore, the strength of the North Korea conclusion would grow in proportion to the number of samples the scientists had from around the world. It is unknown how many samples exist from various uranium deposits or how many samples the federal analysts scrutinized for signs of similarity.
A second nuclear scientist who consults for the federal government on North Korea said he suspected that Pakistani scientists had helped the North ot only to make uranium centrifuges but also to build a plant that transformed raw uranium from the country's mines into uranium hexafluoride, the form that centrifuges can enrich.
He said he didn't think North Korea "did it by itself" ' in making the uranium plant. Another former official suggested that the components for the plant might have been purchased elsewhere, perhaps from Japan or Europe.