North Korea is capable of expanding its nuclear arsenal to a half-dozen weapons within a few years, U.S. intelligence says. In the longer term, U.S. officials fear a full-blown nuclear weapon production line run by the communist country, with the product available to any government willing to pay the price.

Central to the issue is how quickly the North Koreans can produce plutonium or enriched uranium, one or the other of which is necessary to make a nuclear weapon. North Korea has developed programs on both fronts, according to the CIA.

The Bush administration's plans to pursue diplomatic solutions may yet prevent the North from pursuing its nuclear ambitions further. Some U.S. officials have suggested that North Korea's moves are a bluff, intended not to make more weapons but to wrest new economic aid from wealthier nations.

But others say North Korea's covert nuclear weapons program, discovered last summer by U.S. intelligence, shows that the Pyongyang government will try to expand its nuclear arsenal despite any agreements not to do so.

North Korea says it does not want to make new weapons, and U.S. officials acknowledge that it has yet to take some key steps necessary to renew production that was supposed to have been suspended under a 1994 agreement.

U.S. intelligence estimates that North Korea can make a nuclear weapon with between 8.8 pounds and 11 pounds of plutonium, according to American defense officials, speaking on the condition of anonymity. Such a bomb would have a yield of around 10 kilotons, meaning the blast would be equivalent to that from 10,000 tons of TNT.

The CIA believes North Korea probably already has one or two nuclear weapons of this class, built with plutonium manufactured at a small reactor at Yongbyon in the late 1980s. North Korea also has at least four years' accumulation of spent fuel that, once processed, could probably be used to make four or five more weapons within a few years, experts say.

The figures suggest the North Koreans' bomb designs are slightly more advanced than the bomb the United States dropped on Nagasaki, Japan, in 1945. That bomb used 13 pounds of plutonium to produce a roughly 20-kiloton explosion that killed 70,000 people, said Princeton University professor Frank von Hippel, a physicist and former White House security official.

Those designs are nowhere near as powerful as the modern thermonuclear weapons in the U.S., Russian and Chinese arsenals.

The CIA believes North Korea can produce more plutonium by restarting the nuclear reactor at Yongbyon and an adjacent processing facility. But restarting that reactor will take up to a year, and the facility can only produce about 13 pounds of plutonium annually. That is enough for one weapon a year with a little plutonium left over.

Still, a jump to perhaps six nuclear weapons would increase North Korea's capabilities greatly, said Steve Fetter, a professor at the University of Maryland and former Pentagon official who studied North Korea.

One or two weapons are last-resort devices because once fired, the North Koreans no longer would have a deterrent against a nuclear response. But a half-dozen would provide the ability to strike and be ready to strike again.

An expansion of North Korea's arsenal may push Japan and South Korea to build nuclear weapons of their own, potentially starting a regional arms race with China. Defense officials believe North Korea probably would put nuclear warheads on No Dong ballistic missiles, which can hit targets in Japan. North Korea also is working on longer-range missiles.

But longer-term fears center on North Korea making weapons with uranium and starting a large-scale plutonium production effort. North Korea has taken steps to do both, according to the CIA.

Completing two half-finished nuclear reactors, at Yongbyon and Taechon, would allow North Korea to produce as much as 600 pounds of plutonium a year, enough for more than 50 weapons annually.

While the CIA says finishing those reactors would take several years, the prospect still raises fears that North Korea could begin making nuclear weapons for sale to countries such as Iran, Iraq or Libya.

"I doubt North Korea would have any use for more than 100 or so," Fetter said. "North Korea doesn't produce much. Everything it has produced it has sold — missiles, for instance."

The CIA believes that North Korea's uranium program began more than two years ago, and says the North has begun building a processing plant that could refine enough weapons-grade uranium to make at least two weapons a year. That plant will not be operational until mid-decade at the earliest. It is unclear if U.S. intelligence has learned where it is.

Such a plant probably would use more than 1,000 water-cooler-sized centrifuges of Pakistani design to refine the 75 pounds to 90 pounds of uranium necessary to make two weapons, von Hippel said.

A functioning uranium-based program, which does not rely on a nuclear reactor, is also much harder to detect by spy satellites. The U.S. discovery of North Korea's clandestine uranium program touched off the current crisis.