WASHINGTON — After decades of maintaining a minimal nuclear force, China has re-engineered many of its long-range ballistic missiles to carry multiple warheads, a step that federal officials and policy analysts say appears designed to give pause to the United States as it prepares to deploy more robust missile defenses in the Pacific.
What makes China’s decision particularly notable is that the technology of miniaturizing warheads and putting three or more atop a single missile has been in Chinese hands for decades. But a succession of Chinese leaders deliberately let it sit unused; they were not interested in getting into the kind of arms race that characterized the Cold War nuclear competition between the United States and the Soviet Union.
Now, however, President Xi Jinpingappears to have altered course, at the same moment that he is building military airfields on disputed islands in the South China Sea, declaring exclusive Chinese “air defense identification zones,” sending Chinese submarines through the Persian Gulf for the first time and creating a powerful new arsenal of cyberweapons.
Many of those steps have taken American officials by surprise and have become evidence of the challenge the Obama administration faces in dealing with China, in particular after American intelligence agencies had predicted that Mr. Xi would focus on economic development and follow the path of his predecessor, who advocated the country’s “peaceful rise.”
Secretary of State John Kerry arrived in Beijing on Saturday to discuss a variety of security and economic issues of concern to the United States, although it remained unclear whether this development with the missiles, which officials describe as recent, was on his agenda.
American officials say that, so far, China has declined to engage in talks on the decision to begin deploying multiple nuclear warheads atop its ballistic missiles.
“The United States would like to have a discussion of the broader issues of nuclear modernization and ballistic missile defense with China,” said Phillip C. Saunders, director of the Center for the Study of Chinese Military Affairs at National Defense University, a Pentagon-funded academic institution attended by many of the military’s next cadre of senior commanders.
“The Chinese have been reluctant to have that discussion in official channels,” Mr. Saunders said, although he and other experts have engaged in unofficial conversations with their Chinese counterparts on the warhead issue.
Beijing’s new nuclear program was reported deep inside the annual Pentagon report to Congress about Chinese military capabilities, disclosing a development that poses a dilemma for the Obama administration, which has never talked publicly about these Chinese nuclear advances.
President Obama is under more pressure than ever to deploy missile defense systems in the Pacific, although American policy officially states that those interceptors are to counter North Korea, not China. At the same time, the president is trying to find a way to signal that he will resist Chinese efforts to intimidate its neighbors, including some of Washington’s closest allies, and to keep the United States out of the Western Pacific.
Already, there is talk in the Pentagon of speeding up the missile defense effort and of sending military ships into international waters near the disputed islands, to make it clear that the United States will insist on free navigation even in areas that China is claiming as its exclusive zone.
Hans M. Kristensen, director of the Nuclear Information Project at the Federation of American Scientists, a policy research group in Washington, called the new deployments of Chinese warheads “a bad day for nuclear constraint.”
“China’s little force is slowly getting a little bigger,” he said, “and its limited capabilities are slowly getting a little better.”
To American officials, the Chinese move fits into a rapid transformation of their strategy under Mr. Xi, now considered one of the most powerful leaders since Mao Zedong or Deng Xiaoping. Vivid photographs, which were released recently, of Chinese efforts to reclaim land on disputed islands in the South China Sea and immediately build airfields on them, underscored for White House policy makers and military planners the speed and intensity of Mr. Xi’s determination to push potential competitors out into the mid-Pacific.
That has involved building aircraft carriers and submarines to create an overall force that could pose a credible challenge to the United States in the event of a regional crisis. Some of China’s military modernization program has been aimed directly at America’s technological advantage. China has sought technologies to block American surveillance and communications satellites, and its major investments in cybertechnology — and probes and attacks on American computer networks — are viewed by American officials as a way to both steal intellectual property and prepare for future conflict.
The upgrade to the nuclear forces fits into that strategy.
“This is obviously part of an effort to prepare for long-term competition with the United States,” said Ashley J. Tellis, a senior associate at the Carnegie Endowment for International Peace who was a senior national security official in the George W. Bush administration. “The Chinese are always fearful of American nuclear advantage.”
American nuclear forces today outnumber China’s by eight to one. The choice of which nuclear missiles to upgrade was notable, Mr. Tellis said, because China chose “one of few that can unambiguously reach the United States.”
The United States pioneered multiple warheads early in the Cold War. The move was more threatening than simply adding arms. In theory, one missile could release warheads that adjusted their flight paths so each zoomed toward a different target.
The term for the technical advance — multiple independently targetable re-entry vehicle, or MIRV — became one of the Cold War’s most dreaded fixtures. It embodied the horrors of overkill and unthinkable slaughter. Each re-entry vehicle was a miniaturized hydrogen bomb. Each, by definition, was many times more destructive than the crude atomic weapon that leveled Hiroshima.
China watched all this from the sidelines. Gingerly, it improved its warheads and missile forces in what amounted to baby steps, but chose to field a force that the leadership in Beijing believed could deter aggression with the smallest number of deployed warheads.
In 1999, during the Clinton administration, Republicans in Congress charged that Chinese spies had stolen the secrets of H-bomb miniaturization. But intelligence agencies noted Beijing’s restraint.
“For 20 years,” the C.I.A. reported, “China has had the technical capability to develop” missiles with multiple warheads and could, if so desired, upgrade its missile forces with MIRVs “in a few years.”
The calculus shifted in 2004, when the Bush administration began deploying a ground-based antimissile system in Alaska and California. Early in 2013, the Obama administration, worrying about North Korean nuclear advances, ordered an upgrade. It called for the interceptors to increase in number to 44 from 30.
While administration officials emphasized that Chinese missiles were not in the system’s cross hairs, they acknowledged that the growing number of interceptors might shatter at least some of Beijing’s warheads.
Today, analysts see China’s addition of multiple warheads as at least partly a response to Washington’s antimissile strides. “They’re doing it,” Mr. Kristensen of the Federation of American Scientists said, “to make sure they could get through the ballistic missile defenses.”
The Pentagon report, released on May 8, said that Beijing’s most powerful weapon now bore MIRV warheads. The intercontinental ballistic missile is known as the DF-5 (for Dong Feng, or East Wind). The Pentagon has said that China has about 20 in underground silos.
Private analysts said each upgraded DF-5 had probably received three warheads and that the advances might span half the missile force. If so, the number of warheads China can fire from that weapon at the United States has increased to about 40 from 20.
“It’s been a long time coming,” said Jeffrey Lewis, an expert on Chinese nuclear forces at the Middlebury Institute of International Studies at Monterey. In an interview, he emphasized that even fewer of the DF-5s might have received the upgrade.
Early last week, Mr. Kristensen posted a public report on the missile intelligence.
Beijing’s new membership in “the MIRV club,” he said, “strains the credibility of China’s official assurance that it only wants a minimum nuclear deterrent and is not part of a nuclear arms race.”