This article was reported by Douglas Frantz, James Risen and David E. Sanger and written by Mr. Sanger.

he United States is investigating new intelligence reports of contacts between Pakistani nuclear weapons scientists and the Taliban or the terrorist network Al Qaeda, according to Pakistani and American officials.

More than a month ago, Pakistan detained and interrogated two nuclear scientists who had contacts with the Taliban and Al Qaeda, but neither had any knowledge or expertise that would have helped terrorists build or obtain a nuclear weapon, the officials said.

Since then, however, American and Pakistani officials have received new reports of other possible contacts involving scientists with actual experience in production of nuclear weapons and related technology.

The officials in the United States and Pakistan offered different, and sometimes conflicting, accounts of the nature of those contacts and who might be involved. But American officials said the intelligence was credible enough for them to focus new concern on the security of Pakistan's weapons program.

Pakistani officials said their government was resisting some of the American efforts to interrogate several of the scientists and engineers, for fear that the intelligence reports may be a ploy by Washington to learn details of Pakistan's secret nuclear program.

According to Pakistani officials and news reports in Pakistan in recent days, the United States has asked that two other nuclear experts, Suleiman Asad and Muhammed Ali Mukhtar, with long experience at two of Pakistan's most secret nuclear installations, be questioned.

Pakistani officials said George J. Tenet, the director of central intelligence, discussed this issue with top Pakistani officials while he was in the country last weekend. C.I.A. officials would not confirm that account, but White House officials said Mr. Tenet's trip was related in part to nuclear issues.

But in an unusual move, as soon as Mr. Tenet returned to Washington, Pakistani officials volunteered to Pakistani and Western reporters that Mr. Asad and Mr. Mukhtar were the subjects of concern by the C.I.A. The motives of the Pakistani officials for disclosing the information were unclear, but they also said the two men were unavailable because they were sent, shortly after Sept. 11, on a vague research project to Myanmar, formerly Burma, and were not expected home anytime soon.

In fact, one Pakistani official said that Gen. Pervez Musharraf, Pakistan's military president, who met Mr. Tenet during his trip, telephoned one of Myanmar's military rulers to ask him to provide temporary asylum for the two nuclear specialists, offering his assurances that they were not connected to terrorism. A spokesman for Pakistan's Atomic Energy Commission told a Pakistani news service that "we don't want to interrupt them" by returning them to Pakistan for questioning.

While much about this latest dispute remains unclear, it underscores the degree to which Pakistan and the United States are at odds over important issues despite recent cooperation in the war against terrorism.

The United States is concerned that Al Qaeda is trying to obtain at least a primitive radioactive weapon and has concerns about the security of the Pakistani nuclear weapons program, the officials said.

The Pakistani government, for its part, is suspicious that Washington, which is also trying to grow closer to Pakistan's nuclear rival, India, is using its security concerns as a pretext for prying open Pakistan's nuclear weapons program.

Pakistan has always barred international inspectors from examining its facilities or taking stock of its production of plutonium and highly enriched uranium, used to make weapons.

So far, American officials say, the Bush administration does not believe Al Qaeda has a nuclear weapon, despite its clear desire to obtain one. On Friday Gen. Tommy R. Franks, the American commander heading the Afghanistan operations, said, "We have not yet found evidence of weapons of mass destruction in the sites that we have been in."

But officials in Washington remain concerned that Al Qaeda cells elsewhere may be searching for enough material to make a "dirty bomb," in which radioactive material would be wrapped around a conventional explosive and detonated, spreading nuclear contamination.

Two Pakistani nuclear scientists who have been detained and questioned by Pakistan did meet with Taliban and Al Qaeda officials in Afghanistan to discuss nuclear issues. But the scientists, Sultan Bashiruddin Mahmood and Chaudry Abdul Majeed, were not weapons experts, and therefore of little value to terrorists, American officials say.

Under interrogation, Mr. Mahmood and Mr. Majeed have recounted discussions with the Taliban and Al Qaeda, an American official said. The interrogations disclosed that Al Qaeda officials did not have even the most basic knowledge of nuclear weapons and materials, the American official said. "It was the blind leading the blind," the official said.

The interrogations have provided new evidence to suggest that Al Qaeda has been lacking in technical expertise, the official added. "If they had been handed the plans for a nuclear bomb, the worst they could have done is use them as kindling to start a fire," the official said.

But in the interrogations, one of the two scientists mentioned that he had a personal relationship with a Pakistani, and that the man had also been in contact with the Taliban, an American official said. United States intelligence officials believe that they have identified the man as a weapons expert who has left the Pakistani program and is now in business, an intelligence official said. While unable to confirm that account, another American intelligence official said there were new reports suggesting previously undisclosed connections between Pakistani nuclear weapons experts and the Taliban or Al Qaeda.

American and Pakistani officials said that at least some of the scientists the United States is worried about had been involved in the complex of top-secret nuclear facilities southwest of Islamabad where much of Pakistan's rogue nuclear weapons program is concentrated. It remains unclear whether Pakistan plans to detain any of the individuals suspected of involvement.

The new American concern over Pakistan's nuclear program highlights what could well become a growing source of tension between the United States and Pakistan as the war against terrorism enters a new phase. Mr. Bush is more focused than ever, his aides say, on preventing any repeat of the Sept. 11 terrorism, and is particularly worried that Al Qaeda, seeking revenge for the American success in Afghanistan, will use any weapon it can find.

But in private, midlevel Pakistani officials say that while they share Mr. Bush's concern, they also believe that the United States is trying to leverage the current crisis to discover more about Pakistan's facilities, in case Washington someday feels the need to secure or destroy them.

But the American approach, to one Pakistani government official, seems straightforward. Asked in Islamabad about the American requests for cooperation, he characterized the requests this way: "One of the things the U.S. wants is Pakistani knowledge of the market. Could these people have passed on how to acquire technology? Who is selling on the international market?"

If the survivors of the American- led military assault on Al Qaeda in Afghanistan are searching for such nuclear technology and materials, there are two natural targets: Russia and Pakistan. The Pakistani program may be particularly tempting, American officials say, because its major facilities are near the Afghanistan border, as far from India as possible. Pakistan has barred international inspections of the facilities, so their security is unclear.

While American officials believe that Pakistan has built fewer than 20 complete nuclear weapons, all based on designs that use uranium, they also believe that Pakistan has enough weapons-grade material to build a total of at least 45 nuclear weapons. That figure includes Pakistan's recent production of plutonium, enough for at least five bombs.

As one former American official who carefully followed the program until recently said, the estimates of Pakistan's nuclear material are "almost certainly way, way low." The fact of the matter, said another senior Bush administration official in Washington this week, is, "we simply don't know what they've got, how much they've made. That means we can't create a baseline" to determine whether nuclear material is missing.

But the most immediate concern is whether Pakistani scientists and engineers harbor sympathies for the defeated Taliban government in Afghanistan, or are willing to carry on for Osama bin Laden. "Is there loose plutonium in Pakistan?" one senior administration official with lengthy experience in Pakistan said on Friday. "I don't think so. Is there loose technology? That's a different question, and everyone there who has knowledge and access to the material needs to be talked to."

The interrogations of Pakistani scientists and engineers began several weeks ago. After a tip from the United States, Pakistani authorities last month arrested Mr. Mahmood and Mr. Majeed. Both men were associated with a private foundation that did humanitarian work in Afghanistan, and both apparently had contact with Al Qaeda members within the country. Papers found in the foundation's office in Kabul indicated that someone there was also sketching out designs for a helium balloon that could disperse anthrax.

The two men were released and then rearrested, and attempts to reach them have been unsuccessful. They are still being detained without charges. A spokesman for the Pakistani foreign ministry said yesterday that several other associates of the private foundation had recently been detained for questioning, but that none of them were nuclear experts. The families of Mr. Mahmood and Mr. Majeed have said they are innocent of any wrongdoing.

Gary Samore, a senior fellow at the International Institute for Strategic Studies in London and a former senior nonproliferation specialist in the Clinton White House, returned from Pakistan last week with a similar report.

"Pakistani officials claim that no sensitive nuclear materials or information was provided by these retired scientists to Al Qaeda, although they acknowledged that there were discussions that were ongoing," he said. "The critical question is whether that is accurate, and whether there are other cases of individual Pakistani scientists willing to sell nuclear or missile information."

American intelligence officials are increasingly convinced that Pakistan may become the site of a furtive struggle between those trying to keep nuclear technology secure and those looking to export it for terrorism or for profit.

"The Pakistanis themselves have a strong interest in keeping everything locked down," one senior American official said. "But at the same time, they refuse to stop producing new material," because India, Pakistan's nuclear rival, continues its own production. "And there are some in the Pakistani hierarchy who fear a Trojan horse that we are learning about their nuclear program because, in their minds, we may one day need to deal with it."