One month ago, a disturbing report based on satellite imagery showed that China was building about 120 silos for intercontinental ballistic missiles near Yumen in Gansu province, some 1,300 miles west of Beijing. At the time, we raised questions about China’s intentions. Now, a new report has identified a second field taking shape, with about 110 silos near Hami in eastern Xinjiang, 240 miles northwest of the first site. Considering other locations where missile silos are under construction, China seems to be aiming for a tenfold increase in intercontinental ballistic missiles if each silo were filled. What is going on?

In previous years, China insisted that its posture was “minimum deterrence.” It possessed about 200 nuclear warheads, far fewer than the thousands maintained by the United States and Russia. China eschewed keeping missiles on launch-ready alert like the United States and Russia. By all accounts, China poured resources into modernizing conventional or nonnuclear forces. Before the latest disclosures, China had about 100 land-based ICBMs divided among 20 or so silos, with the rest on mobile launchers.

But China’s nuclear ambitions are rising. Matt Korda and Hans Kristensen of the Federation of American Scientists discovered the new missile silo field with satellite imagery from Planet Labs. They found identical domed structures sitting atop silos under construction as were evident at Yumen and at Jilantai, a training site at Inner Mongolia. All told, they say, China now has 250 silos under construction, “the most significant expansion of the Chinese nuclear arsenal ever.” They add, “The number of new Chinese silos under construction exceeds the number of silo-based ICBMs operated by Russia, and constitutes more than half of the size of the entire US ICBM force.” China’s effort is “the most extensive” since U.S. and Soviet missile silo construction during the Cold War, they note.

When the first new field appeared, speculation arose that China might be playing a shell game, moving a few missiles among many silos. But the discovery of a second field throws this theory into doubt. The second field is unsettling evidence of a major Chinese nuclear weapons expansion, which Adm. Charles Richard, head of U.S. Strategic Command, warned of in testimony to Congress in April.

China already is aiming at creating a land-sea-air triad like those of Russia and the United States, and soon it will have the capability of multiple, independently targetable reentry vehicles, or MIRVs. Adm. Richard said China has “moved a portion of its nuclear force to a Launch on Warning (LOW) posture and [is] adopting a limited ‘high alert duty’ strategy.”

An unanswered question is what China thinks it will gain by vaulting to a nuclear posture closer to that of the United States and Russia. The response by the United States and the West is either more nuclear weapons — a new arms race — or nuclear arms control, in which China has not shown much interest. The new missile silos are an ominous sign of a growing challenge, made even more vexing by the other tensions between Washington and Beijing.