In justifying the accelerated deployment of a nationwide anti-missile system, the Bush administration has cited a growing missile threat, particularly from North Korea.

But the extent of North Korea's missile program is open to debate. The U.S. intelligence community concluded several years ago that North Korea had already acquired some kind of long-range missile capability. A number of outside experts, however, doubt that nation has been able to surmount all the technical challenges to firing a nuclear-tipped missile at the continental United States, about 6,000 miles away.

"It would require a huge technological leap for them," said Joseph Cirincione, a specialist in weapons proliferation at the Carnegie Endowment for International Peace. "I don't see the evidence that they've made the necessary breakthroughs."

Complicating any assessment is a shortage of reliable information about North Korea's development effort, given the country's penchant for secrecy. If it has long-range missiles, it has yet to launch one.

Beyond North Korea, the urgency of any new missile threat to the United States is subject to even greater dispute. U.S. officials frequently cite Iran as not far behind North Korea in its pursuit of longer-range rockets. But the most that Iran has demonstrated so far is the Shahab-3, a missile barely capable of reaching southern Europe, let alone the United States.

In general, administration officials say, the spread of ballistic missile technology to about 30 nations has increased the prospects of more countries acquiring the ability to strike at the United States. But these statistics tend to blur the distinction between shorter-range missile arsenals, which are widespread, and longer-range stockpiles, which remain confined to a few advanced countries. Sizable financial as well as technological obstacles confront poorer nations trying to develop long-range missiles.

The last National Intelligence Estimate on the ballistic missile threat, representing the consensus view of all U.S. intelligence agencies, was issued in December 2001. It warned that North Korea had developed a long-range missile known as the Taepo Dong 2 and could start flight-testing it at any time.

Last year, George J. Tenet, who was then director of the CIA, told a Senate committee that North Korea had the ability to strike the West Coast of the United States with a long-range missile, making it a member of a group that for decades had included only Russia, China, Britain and France. Earlier this year, the Pentagon updated its own intelligence assessment of North Korea's missile program, but has declined to discuss any new findings.

Several U.S. officials, interviewed on the condition they not be named, acknowledged large gaps in U.S. knowledge of how advanced the North Korean effort may be. "There are a lot of things that we don't know about the North Korean program," said a senior White House official who follows the issue.

U.S. intelligence analysts have offered some informed judgments, describing two possible, basic versions of the Taepo Dong 2: a two-stage model that could deliver a small nuclear device as far as 6,000 miles -- far enough to reach Alaska, Hawaii and western parts of the United States -- and a three-stage model that could travel 9,000 miles.

An earlier, less powerful version, the Taepo Dong 1, flew for the first -- and so far only -- time six years ago. It soared over Japan in a failed attempt to put a broadcast satellite into orbit.

The following year, North Korea announced a moratorium on flight tests of the Taepo Dong and has held to it since. But ground tests of engines and other improvement efforts have not stopped, U.S. officials and outside specialists said. In recent days, U.S., Japanese and South Korean officials have reported signs that North Korea may be preparing to launch a ballistic missile, although the likely range and purpose of the flight have remained unclear.

"There is some evidence that North Korea has continued to work on the TD-2 or other types of long-range missiles since the August 1998 TD-1 test, but information on the level of effort and the level of success is very fragmentary and elusive," the International Institute for Strategic Studies said in January. "As a result, a firm judgment on the status of the TD-2 cannot be rendered because too little information is available."

One source of heightened U.S. concern has been the emergence in the past year of a new North Korean intermediate-range missile. While not able to reach the United States, the new missile is described by some U.S. and South Korean analysts as having a range of more than 2,000 miles. This would give North Korea the ability to strike Guam, a U.S. territory with a substantial U.S. military presence.

Unlike earlier North Korean missiles, which were derived from Soviet Scud technology, the new land-based missile is said to be based on a more powerful Soviet model, the submarine-launched SS-N-6. U.S. officials worry this technology, which North Korea is suspected of obtaining in the mid-1990s, could become a building block for a new long-range missile.

"The fundamental point is, basically, the North Koreans could decide at any time to flight-test a longer-range system," said Vann H. Van Diepen, a senior State Department official who works on proliferation issues. "They've been in that configuration literally for years."

Once it flies the Taepo Dong 2, North Korea may not wait for more tests to start deploying the missile, U.S. officials say. A decade ago, after flying its medium-range No Dong missile for the first time, North Korea began offering it for sale to other countries.

The launch of the Taepo Dong 1 in 1998 surprised U.S. intelligence agencies by demonstrating that North Korea had developed the ability to achieve multiple staging in a missile flight. But to reach the United States with a nuclear-tipped missile, North Korea would have to overcome other technical hurdles, defense specialists said.

It would need, for instance, to make major advances in propulsion and guidance. It would need to develop a warhead that could withstand the heat and stress of reentry from space. And it would have to produce a nuclear device small enough to fit on the tip of a missile.

"You get as many people arguing that their design can't be that far along as you do people saying yes indeed it can be," said Dennis M. Gormley, senior fellow at Monterey Institute's Center for Nonproliferation Studies.

The fourth rocket designed to intercept warheads headed toward the United States is placed into its underground silo at a new launch site in Fort Greely, Alaska.