By JAMES RISEN and JEFF GERTH, New York Times.

WASHINGTON -- China is close to deploying a nuclear missile with a warhead whose design draws on stolen American secrets, United States intelligence officials say.

A long-range Chinese missile, known as the Dong Feng-31, is being equipped with a small nuclear warhead whose design uses secret American technology, according to American intelligence assessments. The technology is believed to have been stolen from a Government weapons laboratory, although there is some debate over precisely what technical information officials believe is being used.

According to the assessments, the missile is expected to be deployed within three or four years, giving China what officials believe would be its first warhead designed using secret American technology.

Since suspicions of Chinese nuclear espionage became public, the Clinton Administration has said that there is no evidence that Beijing has actually deployed nuclear weapons that rely on stolen American secrets.

Thursday night, the White House declined to comment on the assessments of China's nuclear intentions.

Officials have said, for example, that China stole design information about America's most advanced warhead, the W-88, between 1984 to 1988. Yet they stress that while China has developed a test version with a similar design, it has not actually produced such a weapon.

American officials believe that the technology suspected of having been stolen for use in the DF-31's warhead will help China achieve its goal of building a modern nuclear arsenal that relies on mobility to evade attacks. The DF-31 will be a truck-based mobile missile that can be moved, thus making it more difficult to detect and destroy.

China has denied allegations that it stole United States secrets, and insists that its weapons are based on its own research and development.

China's nuclear arsenal is still much smaller and less technically advanced than that of the United States. Yet the DF-31 and its new warhead represent a step forward in China's efforts to present a more formidable nuclear presence.

Officials say that also means China may soon be using secrets stolen from the United States on weapons capable of a significant range that could include Europe, Asia and possibly the western United States.

American intelligence assessments say the DF-31 will have a range of approximately 5,000 miles. It is expected to be ready for deployment as early as 2002 or 2003.

"The DF-31 ICBM will give China a major strike capability that will be difficult to counterattack at any stage of its operation," a 1996 Air Force intelligence report on the DF-31 stated. "It will be a significant threat not only to U.S. forces deployed in the Pacific theater, but to portions of the continental United States and to many of our allies."

Some United States officials say the new Chinese weapon will use design technology from the American W-70 warhead, a small bomb designed at the Lawrence Livermore National Laboratory in California in the 1970's.

China stole secret design information about the W-70 from the lab in the late 1970's or early 1980's, Government investigators believe. A scientist was fired from Lawrence Livermore in 1981 in connection with the investigation into the suspected theft, but no one has ever been arrested in the case. The F.B.I. said it did not have evidence to bring charges in the case.

The scientist involved in the suspected espionage has never been publicly identified.

Some American officials believe that China used design information from the "primary" of the W-70 to help develop the advanced warhead that will be used on the DF-31 missile. The "primary" of a modern nuclear weapon is a small atomic bomb that serves to ignite the "secondary," a larger hydrogen bomb.

The W-70 warhead is also known as the neutron bomb, a weapon that kills people with enhanced radiation while leaving buildings intact. But its "primary" can be used in other nuclear weapons as well.

American officials have based their belief of Chinese reliance on American design secrets for the new warhead in part on analyses of Chinese nuclear tests in the late 1980's and early 1990's. Officials believe that those successful tests were of a small warhead that is scheduled for use on the DF-31.

Intelligence officials have also based their assessments on analyses of the range and payload abilities of the DF-31.

Because the analyses are based partly on inferences, there is still a debate under way within the American intelligence community over which American nuclear secrets the Chinese are using. There is also disagreement over the extent to which the DF-31's warhead will rely on American technology, officials said.

"There is a debate over how much design information of ours they are using," one official said. "This is a very sophisticated piece of equipment that was difficult for us to develop, so we think it would be hard for them to develop."

Despite the debate, the broad conclusion that the DF-31 will come equipped with a warhead that uses stolen American technology is included in two new secret Government reports, officials said.

Part of one report from a select House committee is soon to be made public. The other report is the result of a Government-wide intelligence assessment of the damage done to United States national security by Chinese nuclear espionage, according to officials. That Government-wide assessment, however, acknowledges the uncertainty.

An unclassified summary of the Government-wide intelligence assessment released last month stated that "it is more likely that the Chinese used U.S. design information to inform their own program than to replicate U.S. weapons designs," in other words, used information to develop their own projects rather than to merely copy American weapons.

The Congressional report does not name the specific warhead design thought to have been stolen by China.

Some officials say that it was not until the Government-wide assessment that many intelligence analysts began to draw connections between the suspected theft of design secrets with the evidence gathered on the DF-31 and the continuing modernization of China's nuclear arsenal.

The DF-31, a solid-fuel missile, would be the first Chinese inter-continental ballistic missile that would be moved on roads. China test-fired its rocket motor last July, and the missile was scheduled to be flight-tested last December, The Washington Times has reported.

Once the DF-31 and other advanced missiles are deployed, China is expected to begin to phase out its older and less accurate ballistic missiles. "The DF-31 ICBM will give China a major strike capability that will be difficult to counterattack at any stage of its operation, from preflight mobile operations through the terminal flight phase," the 1996 Air Force intelligence report predicted.

The "road mobility" of the DF-31, the report adds, "will greatly improve Chinese nuclear ballistic missile survivability and will complicate the task of defeating the Chinese threat."