China has begun construction of more than 100 new intercontinental ballistic missile silos in a desert near the northwestern city of Yumen, according to independent experts, a building spree that could signal a significant expansion of Beijing’s nuclear capabilities.

Researchers at the James Martin Center for Nonproliferation Studies in Monterey, California, obtained commercial satellite images that show work being done at dozens of sites across a grid covering hundreds of square miles of arid terrain in China’s Gansu province. The 119 nearly identical construction sites include features similar to those found at existing launch facilities for China’s arsenal of nuclear-tipped ballistic missiles.

If completed, the acquisition of more than 100 new missile silos would represent a historic shift for China, which is thought to have a relatively modest stockpile of 250 to 350 nuclear weapons. The exact number of new missiles destined for those silos is unknown, but it could be significantly less. In the past, China has used decoy silos.

During the Cold War, the US devised a strategy to move its ICBMs across a grid of silos in a kind of nuclear shell game, ensuring that Soviet war planners never knew where the missiles were at any given time.

According to researcher Jeffrey Lewis, an expert on China’s nuclear arsenal and part of a team that analyzed the suspicious sites, which were first spotted by colleague Decker Eveleth as he scoured photos taken by commercial satellites over northwestern China. The scale of the building spree was described as “incredible” by Lewis.

The discovery comes on the heels of recent Pentagon warnings about China’s rapid advancements in nuclear capability.  Adm. Charles Richard, commander of the United States nuclear forces, stated at a congressional hearing in April that China was undergoing a “breathtaking expansion,” including an expanding arsenal of ICBMs and new mobile missile launchers that can be easily hidden from satellites. Furthermore, the Chinese navy has added new nuclear-weapons-capable submarines to its expanding fleet.

The reported silo construction project could give China yet another way to conceal its most powerful weapons. The construction sites visible on satellite images are spread across two large swaths of a desert basin to the west and southwest of Yumen, a city of 170,000 people on China’s ancient Silk Road.

Each site is approximately two miles apart from its neighbors, and many of the sites are concealed by a large, dome-like covering, a practice observed at known missile silo construction sites in other parts of China. Construction crews can be seen excavating a distinctive circular-shaped pit in the desert floor at sites where the dome is not yet in place. Another building site appears to be a partially completed control center.

Lewis believes the silos are for a Chinese ICBM known as the DF-41, which can carry multiple warheads and reach targets up to 9,300 miles away, potentially putting the US mainland within range. Major excavation work on the sites began early this year, though Lewis believes preparations had been going on for months.

Emails and faxes sent to China’s Foreign Ministry in Beijing and the Chinese Embassy in Washington went unanswered.

Missile silos are easily detected by trained imagery analysts and are vulnerable to destruction by precision-guided missiles in the early stages of a nuclear war. For these reasons, Lewis sees the silo construction project as part of a broader deterrent strategy by a country whose nuclear arsenal dwarfs that of the United States and Russia, which each have over 11,000 nuclear warheads.

Rather than engaging in a costly arms race with Washington and Moscow, China has traditionally adhered to a “limited deterrence” doctrine that prioritizes a lean but robust nuclear arsenal that ensures Beijing’s ability to retaliate against any adversary if attacked.

However, Chinese officials have recently expressed concern that their country’s nuclear deterrent is losing credibility as a result of nuclear modernization programs proposed or already underway in Russia and the United States. Beijing has resisted calls to participate in new arms-control talks, fearing that new limits would permanently cement its status as a second-rate nuclear power in comparison to Washington and Moscow.