A Bush administration intelligence review has concluded that four nations -- including Iraq and North Korea -- possess covert stocks of the smallpox pathogen, according to two officials who received classified briefings. Records and operations manuals captured this year in Afghanistan and elsewhere, they said, also disclosed that Osama bin Laden devoted money and personnel to pursue smallpox, among other biological weapons.

These assessments, though unrelated, have helped drive the U.S. government to the brink of a mass vaccination campaign that would be among the costliest steps, financially and politically, in a year-long effort to safeguard the U.S. homeland. Public health authorities in and out of government project that the vaccine itself, widely administered, could kill more Americans -- 300 is a common estimate, and some are higher -- than any terrorist attack save that of Sept. 11, 2001. It has been left to President Bush to resolve a deadlock among his advisers. Vice President Cheney is said by participants in the debate to be pressing for rapid, universal inoculation, while Health and Human Services Secretary Tommy G. Thompson prefers a voluntary program that would wait at least two years for an improved vaccine.

In public, the White House has described its smallpox concerns in only hypothetical terms, and until now the gravity of its assessment has not been known. Bush administration officials did not share their evidence with a panel of outside scientists established to advise them on smallpox. Some officials said the reticence results from unwillingness to compromise intelligence sources. Others cited fear of provoking public demands for action the government is not yet prepared to take.

Washington's anxiety about smallpox, and limited intelligence-sharing with friendly governments, have prompted urgent requests from allies in the Middle East -- including Jordan and Kuwait -- for assistance in obtaining vaccine before the outbreak of war with Iraq. The National Security Council's Deputies Committee, a panel of officials just below Cabinet rank, met last Tuesday to weigh the allies' requests.

Smallpox, which spreads by respiration and kills roughly one in three of those infected, took hundreds of millions of lives during a recorded history dating to Pharaonic Egypt. The last case was in 1978, and the disease was declared eradicated on May 8, 1980. All but two countries reported by Dec. 9, 1983, that they no longer possessed the virus, but the World Health Organization had no means to verify those reports. Seed cultures are now held officially in only two heavily guarded laboratories, one in Atlanta and the other in Koltsovo, Siberia. The United States renounced germ warfare in 1969 and has undertaken no known offensive program since.

An authoritative official said there is "no reason" to believe bin Laden succeeded in obtaining the smallpox pathogen. Bin Laden's efforts are significant chiefly because U.S. policymakers believe he would use it.

"Al Qaeda is interested in acquiring biological weapons, to include smallpox," according to a classified intelligence summary prepared for senior officials debating options on the scope of a preventive vaccination campaign. Officials who read the homeland security briefing said bin Laden's organization spent money on the effort, but gave higher priority to other biological and chemical agents. The "top five list" for al Qaeda, one official said, included anthrax, the nerve agent ricin, and botulinum toxin.

The U.S. government has known since the early 1990s about Soviet-era smallpox weapons, and collected circumstantial evidence of programs elsewhere. But substantial new reporting has circulated in recent months. "This is not an issue where once every two years we put out an intelligence estimate," one official said. "There's an ongoing requirement to assess the threat. I see reports on this every other week."

The CIA now assesses that four nations -- Iraq, North Korea, Russia and, to the surprise of some specialists, France -- have undeclared samples of the smallpox virus.

The agency's Weapons Intelligence, Nonproliferation and Arms Control Center (WINPAC) described a sliding scale of confidence in those assessments in a briefing prepared last spring. The briefing circulated among senior homeland security, public health and national security officials. Though the quality of its information varied from "very high" to "medium," one official said the report covered only nations for which "we have good evidence."

WINPAC placed Russia in the top category, saying that contrary to diplomatic assurances, Russia retains covert stocks of the virus. The Soviet Union produced smallpox by the ton -- a laborious endeavor, since the standard method is to grow cultures in the lining of chicken eggs. Ken Alibek, who was second in command of "black biology" at Biopreparat before he defected in 1992, said in an interview that he supervised production of the virus in liquid form, suitable for delivery on intercontinental missiles. U.S. officials said they generally accept his account.

Iraq and France are assessed to have smallpox with high, but not very high, confidence.

U.S. officials said the French program is believed to be defensive in nature, and some of them expressed consternation that its inclusion in the WINPAC report was disclosed to a reporter. It could not be learned whether the Bush administration has objected to, or sought information about, the French program. France is one of five members of the U.N. Security Council with a veto, and it is the linchpin of U.S. diplomatic efforts to establish a legal basis for war with Iraq.

Jacques Drucker, who stepped down recently as director of France's National Public Health Surveillance Center, said his country favors research with live smallpox that is forbidden under present conventions. France recently opened one of the world's only Biological Containment Level 4 facilities. Drucker said the Jean Merieux Laboratory in Lyon works with viruses that "could be used for bioterrorist purposes," and mentioned hemorrhagic fevers such as ebola, Marburg and lassa. The lab is "equipped for smallpox," he said, but "I would suspect that if there was variola virus left in France it would be on the military laboratory research facilities."

Some of the evidence on Iraq emerged from unpublished discoveries of the U.N. Special Commission (UNSCOM), which searched for prohibited weapons after the Persian Gulf War. In 1995, David Kelly, a British inspector, led a team to the maintenance shop of the State Establishment for Medical Appliances on the edge of Baghdad. There he found a freeze drier labeled "smallpox." Two years later, on Oct. 7, 1997, inspector Diane Seaman seized a document on the grounds of the Al Rasheed Military Hospital describing vaccines currently in use for Iraqi troops. Third on the list was smallpox. Confronted with other evidence on pox research, Iraq's chief bioweaponeer, Hazem Ali, told UNSCOM inspectors that he had considered camelpox as a weapon because Iraqis, unlike Americans, spent enough time near camels to be immune.

Richard Spertzel, UNSCOM's chief biological inspector, said that explanation was laughable. "Only one person ever died of camelpox," Spertzel said in an interview. Ali was "much too good a scientist to believe the story."

On Jan. 14, 1991, the Defense Intelligence Agency said an Iraqi agent described, in medically accurate terms, military smallpox casualties he said he saw in 1985 or 1986. Two weeks later, the Armed Forces Medical Intelligence Center reported that eight of 69 Iraqi prisoners of war whose blood was tested showed current immunity to smallpox, which had not occurred naturally in Iraq for 20 years. The same prisoners had been inoculated for anthrax, a well-established Iraqi bioweapon.

More recently, according to the WINPAC report, a former Soviet scientist told U.S. officials that his country "transferred [smallpox] technology in the early 1990s to Iraq." Northern Iraq suffered one of the last known smallpox epidemics in 1971-72. The WINPAC report assessed that Iraq "retained samples from the 1971 outbreak."

The last country on WINPAC's list is North Korea, which the authors wrote "has a longstanding and active biological weapons program." Though assessing that Pyongyang has the smallpox pathogen, WINPAC said its evidence was of "medium" quality.

On March 5, 1993, the Russian Foreign Intelligence Service reported that "North Korea is performing applied military-biological research" with "pathogens for malignant anthrax, cholera, bubonic plague and smallpox." Gordon Oehler, then head of WINPAC, told Congress that the Russian report was "not a bad summary." Much more recently, sources said, the United States has obtained reports of ongoing pox research and manufacture of vaccine.

"I've spent a lot of time trying to understand the biological weapons threat," one policymaker said in an interview, "and I have concluded on a very personal basis that there is a small chance that we will have definitive evidence, smoking gun evidence, for countries like North Korea, very closed societies."

Confidence about the smallpox evidence varies somewhat among the 14 U.S. intelligence agencies and departments.

"The assessment is, they have it," said one official, speaking as he held his own office's written summaries of evidence on North Korea and Iraq. "We don't say 70 percent certainty. We assess that they have it."

Officials who agreed that the evidence is not decisive said few differences exist in the ultimate judgment of national security and homeland defense officials. One person who has access to the compartmented intelligence on smallpox offered to "bet my next year's salary" that the four countries named in WINPAC's report have live seed cultures.

Bush administration officials with central roles in smallpox policy said the government-commissioned Advisory Committee on Immunization Practices was unequipped for its ostensible role of balancing the risks of vaccination against the risks of a smallpox attack. The committee recommended against a broad vaccination campaign, but many members said they would change their views if they knew a rogue nation possessed the virus.

"They give the scientific assessment of what the risks of vaccination are," a senior administration official said. "They do not have the same amount of information that is circulated around this issue here."

Those who disclosed the intelligence assessments described above, speaking on condition of anonymity, were not authorized by the White House to do so. Those assigned to speak for the administration's views, who also declined to be identified, would not discuss intelligence reports. They hewed to their public position, as one of them put it, that "there is a concern with regard to North Korea and Iraq that they may have smallpox."

U.S. allies' smallpox fears come in part from U.S. reports and -- especially in Jordan -- from independent intelligence on the Iraqi threat. In an interview, Kuwaiti ambassador Salem Abdullah Jaber Sabah acknowledged that his government asked for vaccine last summer "in readiness for any eventuality."

Two U.S. officials called the requests unlikely to be granted. The scarcity of vaccine, and likely repercussions in domestic and coalition politics, permit Bush to do no more, they said, than offer assurances of help if Iraq's neighbors suffer an outbreak.

Cheney, who confronted biological threats as defense secretary years ago, was energized about smallpox by a videotape and briefing shortly after Sept. 11, 2001. In a war game called Dark Winter, former senator Sam Nunn played a president who failed to contain a fictional smallpox outbreak that began in Oklahoma City. It spread in less than two weeks to 25 states and 15 countries overseas, inflicting "massive civilian casualties."

"It's a dramatic briefing," Cheney's chief of staff, I. Lewis Libby, recalled, "but we were well on this road already." Libby said Cheney favors "a forward-leaning position on protecting Americans from this threat," but declined to describe his advice to the president.

At Health and Human Services, officials said, Thompson has been influenced by doubts at the Centers for Disease Control and Prevention.

"If you look at the vice president's office, they're thinking strategic, not public health," said one debate participant. He cited the swine flu debacle of 1976, when President Gerald Ford had to abandon plans for universal inoculation after people starting dying of the vaccine and others developed Guillain-Barre syndrome, a rare and occasionally fatal paralysis. "If something bad happens, the public is not going to be blaming Dick Cheney, they're going to be blaming Tommy Thompson. And the fact is they're going to be blaming the president. That's why the political people are weighing in, and that's why the decision is still sitting on his desk."

Staff researcher Lucy Shackelford contributed to this report.

Thomas C. Marbury inserts a needle into saline solution last month in Miami, as public health care workers were trained to give smallpox vaccinations.