UNITED NATIONS (AFP) - Nuclear physicists on April 26 disputed a claim by U.S. officials that North Korea was close to developing a missile capable of hitting the United States with atomic weapons.

The claim has been used to support arguments that the United States needs a missile defense system to intercept and destroy missiles fired against it by "rogue states."

David Wright, a weapons expert at the Massachusetts Institute of Technology, said in a telephone interview that a rocket like the one North Korea tested in 1998, "might be capable of reaching Alaska with a very small payload of no more than 100 kilograms (220 pounds)."

He doubted it could reach Hawaii, and added that such a missile would be incapable of reaching either state, let alone the continental United States, with a nuclear warhead weighing between 50 (1,100 pounds) and 1,000 kilos (2,200 pounds).

"All the North Koreans have done is fire a few things into the air," said Zia Mian, of the Center for Energy and Environmental Studies at Princeton University.

"That is a long way from developing an ICBM" or Inter-Continental Ballistic Missile, he said in a telephone interview.

North Korea launched a three-stage Taepo Dong I missile over the Sea of Japan on August 31, 1998, attempting to put a small satellite into orbit.

Mian had been asked to comment on a claim by Jesse Helms, chairman of the U.S. Senate Foreign Relations Committee, that "both North Korea and Iran are within five years of possessing viable ICBMs capable of striking the continental United States."

In a statement in Washington, Helms said "North Korea may already have the capacity to strike Alaska and Hawaii."

The U.S. government acknowledges that deployment of a missile shield which it is currently testing is prohibited by the Anti-Ballistic Missile (ABM) treaty which the United States signed with the Soviet Union in 1972.

President Bill Clinton’s chief adviser on arms control and international security, John Holum, told reporters here on April 25 that the U.S. government hoped Russia would agree to modify the treaty.

Holum said the proposed shield was not designed to make Russia’s nuclear deterrent ineffective, but to protect the United States from attack by rogue states.

"The intelligence analysis is that North Korea is very close to an ICBM capability," Holum said.

Wright told AFP that North Korea was still using "low-level technology" of the kind used in Russian-made SCUD missiles.

It used stable liquid propellants rather than solid fuels or cryogenic substances such as liquid oxygen which give greater thrust, he said, and this too was evidence of its lack of sophistication, he said.

It was difficult to increase the ratio of fuel to body weight necessary to convert a SCUD missile to long-range use, he said.

Wright is a member of the Union of Concerned Scientists, which is opposed to deployment of the U.S. missile shield.

"If a country like North Korea had a nuclear weapon and wanted to use it against the United States, it is unlikely that it would use a missile," he said.

It could not carry out many missile tests without advertising its intentions, he said, and that would mean entrusting a valuable payload to an unreliable rocket.

Such a country would more likely disguise a warhead and deliver it by ship, he said.

Mian asked "how many times do you have to seriously test an ICBM capacity before you would entrust your country’s security with it? At least a dozen successful test would be needed to show you were serious."

Stephen Schwartz, editor of The Bulletin of Atomic Scientists, said North Korea’s launch facilities were primitive.

The Bulletin’s latest issue contains a satellite photo of a missile launch site near the Sea of Japan.

"It’s basically a dirt road," Schwartz said in a telephone interview from Cambridge, Massachusetts.

He added that "people we know who have been to North Korea say their equipment is one-of-a-kind, handmade stuff. That is terrible for quality control. How can you be sure a rocket will work if it is not like the one you have tested?"