North Korea, which says it might resume missile tests, could be ready to test a two-stage rocket capable of reaching Alaska or Hawaii with a nuclear weapon-sized payload, according to U.S. defense analysts.

U.S. officials say North Korea is the world's No. 1 proliferator of missile technology, and the threat it poses is one reason why Washington plans to build a limited missile defense system by the end of 2004.

U.S. defense experts say North Korea has one or two nuclear bombs, as well as chemical and biological weapons that can be deployed in warheads.

Fears that North Korea will go ahead with a missile test rose over the weekend, when its ambassador to China threatened new tests if the United States doesn't take steps to improve ties.

The statement by Ambassador Choe Jin Su followed North Korea's withdrawal a day earlier from the Nuclear Nonproliferation Treaty in a dispute with the United States and its allies over its nuclear weapons development.

"Because all agreements have been nullified by the United States side, we believe we cannot go along with the self-imposed missile moratorium any longer," Choe said.

Along with the withdrawal from the non-proliferation treaty, Choe's comment appeared to be part of an effort to pressure the United States into negotiations.

North Korea wants a nonaggression treaty and economic aid from Washington, and views its arsenal of up to 700 missiles as a way to gain political leverage.

North Korea shocked the region in 1998 by test-firing a Taepodong-1 missile over Japan and into the Pacific. The North said it was an attempt to insert a satellite into orbit.

Communist technicians are believed to be working on the more advanced Taepodong-2. U.S. defense experts believe that the missile, if deployed, could deliver a payload of several hundred pounds as far as Alaska or Hawaii, and a lighter payload to the western half of the continental United States.

Technical difficulties and economic hardship have hampered North Korea's missile programs in recent years, and it is unclear whether the Taepodong-2 is ready for testing. If so, it could be done with relatively little warning.

A big challenge for North Korean engineers is the construction of a reentry vehicle for their long-range missile, said Daniel Pinkston, an analyst at the Monterey Institute of International Studies in California.

"It's easier to launch a satellite, but to have a warhead reenter the atmosphere intact and work is more difficult to do," he said.

He speculated that North Korea could choose to test the Taepodong-2 around the Feb. 16 birthday of North Korean leader Kim Jong Il.

Kim has said that regular rocket launches are "vastly uneconomical" for his impoverished country, but a missile test would enhance his prestige at home as a "high-tech" leader.

A Taepodong-2 launch would likely occur at the Musudan-ri test site in an isolated area on North Korea's east coast. The last ballistic missile test prior to 1998 was in 1993, and both were held at that site.

A large chunk of North Korea's foreign exchange earnings comes from the export of missiles and their technology and components, much of it to the Middle East. The U.S. military estimates that the North made $560 million from missiles sales in 2001.

Last month, a North Korean vessel carrying Scud missiles bound for Yemen was intercepted and then released after contacts between the United States and Yemen.

North Korea has also sold missile-related material to Iran, Libya, Syria and Egypt, according to CIA reports. Western media have said that Pakistan gave nuclear secrets to North Korea in return for missile technology. Pakistan denies it.

In talks with the Clinton administration, Pyongyang asked for $1 billion in aid each year for three years in return for stopping missile exports, but a deal was never signed.