The recent disclosures of secret nuclear facilities in Iran and North Korea -- combined with the North's threat this week to resume plutonium production -- have presented the United States with its most serious nuclear challenge since the early 1990s. The episodes have not only forced a reassessment of when the two countries could become nuclear powers but also exposed widening gaps in the international fire walls built decades ago to halt the spread of nuclear materials and technology, weapons experts say.

U.S. officials had long suspected Iran and North Korea of quietly seeking uranium-based nuclear arms. But what was most startling about the revelations of the past few weeks was how much the two countries managed to achieve before anyone noticed, the experts added.

For example, Iran's secret nuclear program was disguised for two years as a water irrigation project in the country's northern desert. Two weeks ago, satellite photos revealed construction near the town of Natanz that U.S. officials say apparently is designed not for pumping water but for enriching uranium.

North Korea agreed in a 1994 pact with the Clinton administration to stop pursuit of a plutonium bomb. But then it created a hidden uranium program and disguised it so well that intelligence officials are still not sure of its location. Accounts by defectors in a recent congressional report point to at least one underground factory in tunnels in Mount Chonma, on the Chinese border. Production of enriched uranium, which would be necessary to make a weapon, appears to be underway, according to the defectors cited by the Congressional Research Service.

The disclosures have spawned new worries that other countries will be drawn into an accelerating arms race just as the Bush administration prepares for a possible conflict with Iraq. The United States has accused Iraq of trying to develop weapons of mass destruction, which Iraq has denied. While the scope of any Iraqi nuclear program is still not known, U.S. officials acknowledge that, if it exists, it is probably far less advanced than those in Iran or North Korea.

"For everyone who hoped that nuclear weapons were somehow receding from international politics, we're now seeing them come back again, in part because of our own failed policies," said Graham Allison, director of Harvard University's Belfer Center for Science and International Affairs. "If North Korea becomes a nuclear state, you can predict that in short order South Korea and Japan may become nuclear states also. After that you've got a devil's brew."

"Just try to imagine," Allison added, "what the Middle East will be like with another nuclear actor."

Even before the recent disclosures, many weapons experts were alarmed by nuclear tests conducted by India and Pakistan in May 1998. The experts have also expressed concern about recent U.S. willingness to consider new uses for nuclear bombs, such as the destruction of heavily fortified bunkers.

"The nuclear issue is back again in a way it hasn't been around since the 1950s," said Andrei Kokoshin, a Russian legislator and an adviser to former president Boris Yeltsin on military and security issues. "There is a great probability that arsenals will grow and new countries will acquire weapons. And we are simply not prepared for it."

In the early 1960s, President John F. Kennedy's advisers made fearful predictions of a world perpetually on the brink, as nuclear weapons and know-how spread to dozens of nations on every continent. But in the decades since, membership in the nuclear club has been restrained, thanks to a combination of international monitoring, superpower pressure and strict controls on the export of sensitive technology and material.

Today, in addition to the original five nuclear powers -- the United States, Russia, Britain, France and China -- only India and Pakistan have declared arsenals of nuclear weapons. Israel is widely assumed to have the bomb, and North Korea is believed to have one or two nuclear devices, according to CIA analysts. South Africa built a bomb in the 1970s but later renounced its nuclear program.

Other nations have sought nuclear weapons, including Iran, Iraq and North Korea. But the technical difficulties inherent in creating fissile material -- plutonium or enriched uranium -- combined with restrictions on nuclear-related exports, helped put the bomb out of their reach. Although clandestine development of nuclear weapons was possible, as Iraq demonstrated in the early 1990s with its crash program to build a bomb, Western intelligence agencies were proficient at spotting the distinctive nuclear reactors and large reprocessing facilities required for making plutonium-based weapons.

Strikingly, both North Korea and Iran managed to fool Western spy satellites by apparently choosing uranium as their fissile material. European technology for enriching uranium for bombs has spread globally in recent years. The technology requires less production space and thus is easier to conceal, weapons experts and intelligence officials say.

"With plutonium you have big production reactors and lots of signs and signals that give you away," said Rose Gottemoeller, formerly deputy undersecretary for defense nuclear non-proliferation in the Department of Energy and now a senior associate at the Carnegie Endowment for International Peace. "It is possible to build a uranium plant without giving off any signals to the outside world."

In addition, both countries appear to be benefiting from relationships with other countries that possess nuclear know-how and are increasingly willing to share it, weapons experts said.

"The spread of enrichment technology was predicted 25 years ago, and now it seems to be happening," said Leonard S. Spector, a deputy director of the Center for Nonproliferation Studies at the Monterey Institute of International Studies. "There seems to be networking among the bad guys -- the technology holders who are perceived as proliferation threats. They're not just keeping it at home, they're sharing it. We haven't seen that before."

U.S. intelligence officials believe North Korea obtained uranium-enrichment technology and equipment from Pakistan in exchange for missiles. The reclusive North Korean government, which had halted its pursuit of a plutonium bomb under the agreement with the Clinton administration, is believed to have begun secretly building a uranium enrichment plant in the late 1990s using hundreds of fast-spinning devices known as gas centrifuges. Pakistan has denied aiding North Korea's nuclear efforts.

In late September, the North Koreans acknowledged the existence of a secret uranium program after Assistant Secretary of States James A. Kelly confronted them with evidence during a meeting in Pyongyang. Tensions have risen in recent weeks, culminating in North Korea's decision to rescind its agreement not to develop plutonium bombs.

If North Korea begins full production of nuclear weapons, it could develop up to five plutonium bombs from its existing stocks of reactor fuel, and could begin production of uranium-based weapons as early as 2004, according to a recent analysis by the Washington-based Nonproliferation Policy Education Center.

Iran's suppliers are less well-known, although U.S. intelligence officials suspect the Tehran government received help from Russian and Ukrainian companies, and possibly from China. The evidence of Iran's program came in the form of commercial satellite photos depicting two suspicious construction projects. One of them -- the "desert eradification" project near the town of Natanz -- has all the markings of a uranium enrichment plant, including eight-foot concrete outer walls to protect the facility against an attack, said David Albright, a former nuclear inspector for the International Atomic Energy Agency, the U.N.-chartered agency that monitors nuclear facilities in scores of nations. The Natanz site and another facility near the town of Arak were first reported by opponents of the Iranian government outside the country in August.

Albright said he believes that strengthened international inspections requested by the IAEA in the 1990s could have detected the facilities sooner, and might prevent others from being developed.

"There's nothing that Iran is doing that would not be caught under [enhanced] inspections," said Albright, whose nonprofit Institute for Science and International Security released the satellite photos.

Other weapons experts say current international controls on proliferation are inadequate to prevent the kinds of violations committed by Iran and North Korea. Not only do the rules allow cheating, but they offer few tools for dealing with problem states, said Henry D. Sokolski, director of the Nonproliferation Policy Education Center. For example, it is currently difficult to prevent such nations as Iran from acquiring the capacity to develop nuclear weapons as long as they do not cross the line into production, he said.

"There is no handbook, no clear enforcement features in the treaties," Sokolski said. "Now that we have, or are about to have, violations, we have to decide what to do. And what we decide to do today will decide what, if anything, will be done with future violators -- and indeed, the fate of the treaties being violated."

This January satellite image shows progress on an Iranian reactor that Iranian TV reports recently described as preparing to go on line.