China has accelerated an expansion of its nuclear arsenal because of a change in its assessment of the threat posed by the U.S., people with knowledge of the Chinese leadership’s thinking say, shedding new light on a buildup that is raising tension between the two countries.

The Chinese nuclear effort long predates Russia’s invasion of Ukraine, but the U.S.’s wariness about getting directly involved in the war there has likely reinforced Beijing’s decision to put greater emphasis on developing nuclear weapons as a deterrent, some of these people say. Chinese leaders see a stronger nuclear arsenal as a way to deter the U.S. from getting directly involved in a potential conflict over Taiwan.

Among recent developments, work has accelerated this year on more than 100 suspected missile silos in China’s remote western region that could be used to house nuclear-tipped missiles capable of reaching the U.S., according to analysts that study satellite images of the area.

American leaders have said the thinking behind China’s nuclear advance is unclear. Independent security analysts who study nuclear proliferation say they are also in the dark about what is driving Beijing after exchanges between Chinese officials and analysts mostly dried up in the past few years.

The people close to the Chinese leadership said China’s increased focus on nuclear weapons is also driven by fears Washington might seek to topple Beijing’s Communist government following a more hawkish turn in U.S. policy toward China under the Trump and Biden administrations.

American military officials and security analysts are concerned China’s nuclear acceleration could mean it would be willing to make a surprise nuclear strike. The people close to the Chinese leadership said Beijing is committed to not using nuclear weapons first.

China plans to maintain an arsenal no larger than necessary to ensure China’s security interests, they said, adding that the Chinese military believes its nuclear weapons are too outdated to present an effective deterrent against a potential U.S. nuclear strike.

“China’s inferior nuclear capability could only lead to growing U.S. pressure on China,” one person close to the leadership said.

Nervous international reaction to Russian leader Vladimir Putin’s

call for his nuclear forces to be put on alert following his invasion of Ukraine has offered Chinese officials a real-world lesson about the strategic value of nuclear weapons. So did Ukraine’s decision in 1994 to turn over the nuclear weapons left in the country after the breakup of the Soviet Union in return for security assurances from the U.S. and Russia.

“Ukraine lost its nuclear deterrence in the past and that’s why it got into a situation like this,” said a retired Chinese military officer with ties to the country’s nuclear program.

Early construction in June 2021 at a site near Yumen, China, believed to be one of 120 underground silos to potentially hold nuclear-tipped missiles.

Photo: Planet Labs PBC

Construction on the suspected missile-silo site near Yumen, China, appeared to be advancing in July 2021.

Photo: Planet Labs PBC

The people familiar with the Chinese leadership’s thinking said Beijing hasn’t conveyed any adjustments to the country’s nuclear policy as a result of developments in Ukraine. China’s Ministry of Defense didn’t respond to a request for comment.

The people have knowledge of Beijing’s thinking about nuclear policy through their work with various agencies involved in security issues. None are directly involved in the setting of nuclear policy. They didn’t preclude that future developments might change Beijing’s approach and said other factors may also be influencing the leadership’s approach to nuclear weapons.

Their observations nevertheless bring greater clarity to a shift in Beijing’s thinking that has far-reaching consequences globally. Rising tension between the U.S. and China over nuclear weapons could throw the world back into a Cold War-style nuclear standoff similar to that seen in the decades following World War II between the U.S. and Soviet Union.

The risk of miscalculations this time could be higher, however, because while the U.S. and Soviet Union communicated about their nuclear weapons during arms control talks from the late 1980s, the Chinese program and Beijing’s thinking on the role of nuclear weapons has been shrouded in secrecy. China has declined to engage in nuclear arms control talks with the U.S., saying Washington should first reduce its nuclear inventory.

U.S. government and private sector estimates put China’s nuclear arsenal in the low hundreds of warheads, far below the roughly 4,000 warheads held by both Russia and the U.S. The Pentagon says it now expects China to have 1,000 warheads by the end of this decade.

Satellite images taken during January show the last 45 of the temporary covers over each of 120 suspected missile silos near the city of Yumen have been removed, suggesting the most sensitive work at all of the silos has been completed, said Matt Korda, a senior research associate for the Nuclear Information Project at the Federation of American Scientists in Washington. At two other smaller silo fields in western China, work is at earlier stages.

The silos at each of the sites are large enough for a new long-range Chinese missile known as the DF-41 that was put into service in 2020 and is capable of hitting the U.S. mainland, analysts say. Tests of missiles that are launched from aircraft and can carry nuclear warheads also give Beijing a stronger chance of being able to retaliate if it is hit first in a nuclear attack.

Work on the suspected missile-silo site near Yumen, China, appeared to be broadening in September 2021.

Photo: Planet Labs PBC

The removal of a temporary cover from the suspected missile silo in Yumen, China, pictured in February 2022, suggests the most sensitive work was completed, military analysts say.

Photo: Planet Labs PBC

In public, China has played down its nuclear pursuits.

“On the assertions made by U.S. officials that China is expanding dramatically its nuclear capabilities, first, let me say that this is untrue,” Fu Cong, director general of the Foreign Ministry’s arms control department, said earlier this year. He said that China is working to ensure its nuclear deterrent meets the minimum level necessary for national defense.

Chinese leaders had seen nuclear weapons as being of limited value because they don’t offer realistic options for fighting most wars. A major shift occurred in early 2020, according to the people familiar with the leadership’s thinking, as the U.S. government hardened its stance toward Beijing in the early stages of the Covid-19 pandemic.

Strong criticism of the Communist Party from senior Trump administration officials spurred a consensus among Chinese leaders that Washington was willing to take greater risks to stop China’s rise, some of the people said.

A May 2020 speech in Mandarin by former Deputy National Security Adviser Matt Pottinger was particularly alarming, they said. Speaking on the anniversary of a pivotal 1919 student protest in China, Mr. Pottinger said: “Wasn’t the goal to achieve citizen-centric government in China, and not replace one regime-centric model with another one? The world will wait for the Chinese people to furnish the answers.”

“The speech was obviously calling the Chinese to topple the Communist Party,” one person familiar with the Chinese leadership’s thinking said.

In response to a request for comment, Mr. Pottinger said that such an interpretation was “a profound admission that the Communist Party knows it has failed to deliver citizen-centric governance, and it confirms what everyone already suspected: What Beijing fears above all is its own people.”

At the same time, increased support from the U.S. for Taiwan, a democratically self-ruled island that Beijing views as a part of China and has vowed to put under its control, prompted Chinese leaders to debate the prospect that the U.S. might be willing to use nuclear weapons in a conflict over the island, according to the people close to the leadership.

Foreign Ministry official Fu Cong says China is working to ensure that the country’s nuclear deterrent meets the minimum level necessary for national defense.

Photo: Ng Han Guan/Associated Press

There are no indications that a war over Taiwan is imminent, but leaders in both the U.S. and China believe the island is the flashpoint most likely to spark military confrontation. The U.S. maintains a policy of not saying whether it would intervene to support Taiwan, an approach intended to deter conflict. If the U.S. did intervene in a war, American bases in Japan, Guam and elsewhere in the western Pacific could become targets for the Chinese military.

Under a review of nuclear policy conducted by the Trump administration in 2018, the U.S. said it might use nuclear weapons to respond to significant nonnuclear attacks on the U.S. or its allies. President Biden is set to stick to that approach, according to U.S. officials.

Beijing believes Washington would consider attacking mainland China with nuclear weapons because the U.S. military faces challenges in defending Taiwan against the People’s Liberation Army using conventional weapons, the people said.

Pointing to Ukraine, some security analysts have suggested China could use the threat of a nuclear strike to deter the U.S. not just from using its own nuclear weapons in a conflict over Taiwan, but from joining the fight with conventional military power as well.

Despite China’s no-first-use pledge, U.S. military officials and analysts fear the PLA could be tempted to use its midrange missiles to wipe out U.S. military bases in the Asia-Pacific region in the event of a war.

“Large-scale conventional military involvement over Taiwan could quickly lead one side or the other to talk themselves into thinking nuclear use may improve the situation for their side,” said Christopher Twomey, an associate professor of National Security Affairs at the U.S. Naval Postgraduate School in Monterey, Calif.

Satellite images show that work began on the Yumen silo field between March and October 2020, not long after the leadership’s concerns about U.S. intentions rose.

China has declined to answer questions about whether the sites are missile silo fields, but in a meeting with senior military officers in March last year, President Xi Jinping urged them to “accelerate the construction of advanced strategic deterrent systems,” state media reported, a reference to nuclear weapons.

““No matter how the situation develops in the future, the world will be more confrontational. Under such circumstances, China definitely needs to maintain nuclear deterrence.””

— A retired Chinese military official

Other recent developments suggest China is putting a greater emphasis on being able to respond to a nuclear attack in kind. Beijing is building an early-warning system to detect incoming missiles with Russian assistance, according to the Pentagon, and in February 2021, China launched a satellite that some analysts believe is the start of a space-based sensor system for missiles.

China is also developing more advanced weapons that could potentially carry nuclear warheads, including hypersonic missiles, which the U.S. has no proven defenses against.

“All of these capabilities work together to say to the U.S.: ‘There is no world in which you can engage in a nuclear first strike against China and not expect nuclear retaliation back on your cities, even with your missile defenses, even with your great counterforce capabilities,’” said Caitlin Talmadge, an associate professor of security studies at Georgetown University.

Uncertainties abound despite the mounting evidence of China’s nuclear buildup. U.S. intelligence estimates of China’s nuclear stockpile tend to fluctuate wildly, a reflection of the challenge in collecting reliable numbers. Meanwhile, analysts caution that China is likely to load only some of its silos with missiles, and only some of those may be nuclear-tipped.

With war continuing to smolder in Ukraine, however, the calculus in Beijing is growing clearer.

“No matter how the situation develops in the future, the world will be more confrontational,” said the retired Chinese military official. “Under such circumstances, China definitely needs to maintain nuclear deterrence.”

Write to Alastair Gale at alastair.gale@wsj.com