Pentagon says China could double nuclear weapons in decade

China’s stockpile of nuclear weapons numbers in the “low 200s,” the Pentagon told Congress Tuesday, providing its most precise estimate to date of Beijing’s nuclear capabilities.

The number is a fraction of the nuclear warheads that the U.S. and Russia have deployed and keep in their stockpiles.

But the Pentagon report projected that China’s nuclear arsenal will at least double in the next decade as Beijing expands and updates its triad of land-based missiles, missile-carrying submarines and bombers.

“China’s nuclear forces appear to be on a trajectory to exceed the size of a ‘minimum deterrent’ as described in the PLA’s own writings as a small quantity of nuclear weapons to strike enemy urban targets,” said the Pentagon report, using the acronym for the People’s Liberation Army.

“Over the next decade, China’s nuclear warhead stockpile—currently estimated to be in the low 200s—is projected to at least double in size as China expands and modernizes its nuclear forces,” it added.

President Trump has long insisted that any future arms control treaty cover Chinese and Russian nuclear warheads. Marshall Billingslea, the chief U.S. arms control negotiator, asserted in June that Beijing was involved in a crash nuclear buildup and might be trying to achieve “some form of nuclear parity” with the U.S. and Russia.

The report noted that China probably already possesses sufficient fissile material to double its warhead stockpile without producing more plutonium or enriched uranium. It also said that China has built new silos south of Mongolia that may be intended for deployment of a new solid-fueled intercontinental ballistic missile dubbed the DF-41. There also are some indications that China may be building additional silos in its Henan province for its D-5 liquid-fueled missile, as well, the report said.

Experts who reviewed the Pentagon assessment—an annual report to Congress on military developments in China—noted that its estimate of China’s nuclear stockpile remains small.

“While China is modernizing its nuclear arsenal, the biggest news is that its nuclear stockpile is smaller than has been widely assumed,” said Hans Kristensen of the Federation of American Scientists.

The U.S., for example, has 1,750 nuclear warheads deployed on its long-range and shorter-range systems, among the 3,800 nuclear weapons in its stockpile, the FAS said. Russia has 1,572 deployed nuclear warheads among the 4,310 in its stockpile.

China has long declined to disclose the size of its nuclear stockpile. China also has declined to join arms control talks with Russia and the U.S., arguing that its capabilities are modest compared to those nations.

In recent weeks, the Trump administration has signaled that it was pursuing a new approach in which it would try to work out a broad “framework” agreement on arms control issues with Moscow and try to bring Beijing on board later.

The administration’s new negotiating strategy could make it possible to reach an election-year agreement between the U.S. and Moscow—a possibility Secretary of State Mike Pompeo alluded to Monday in a radio interview.

“We’re in detailed discussions with them on an arms control agreement,” Mr. Pompeo said, referring to talks with the Russians. “I hope we could get that done before the end of the year.”

Write to Michael R. Gordon at michael.gordon@wsj.com and Nancy A. Youssef at nancy.youssef@wsj.com