The Pentagon has confirmed a report that China’s newest nuclear-powered attack submarine sank in the spring, dealing a major blow to Beijing as it seeks to showcase its growing military prowess amid its rivalry with the United States.

A U.S. Defense Department spokesperson told The Japan Times on condition of anonymity that the submarine sank pier-side.

“It is unclear if it was carrying nuclear fuel at the time,” the spokesperson said Friday, without offering further details.

The sinking was first reported by The Wall Street Journal. It said that the first-in-its-class nuclear-powered submarine sank in May or June at a shipyard in the inland city of Wuhan. Experts believe the sub was likely carrying nuclear fuel at the time.

Neither the Chinese government nor the military has acknowledged the sinking, which was believed to have been quickly covered up by authorities.

But commercial satellite imagery of the site posted to social media over the summer and analyzed by Thomas Shugart, a former U.S. submarine officer and an adjunct senior fellow at the Center for a New American Security, showed large floating cranes had arrived in early June, apparently to salvage the sub from the bed of the Yangtze River. The site is home to a shipyard of China State Shipbuilding Corp., the state-owned builder of the sub.

The Chinese People’s Liberation Army Navy has worked tirelessly in recent years to expand its reach, outpacing its rivals — including the United States Navy — in building the world’s largest navy, with more than 370 vessels. According to the Pentagon, this force is expected to grow to 395 ships by 2025 and 435 ships by 2030.

The Chinese Navy has in particular placed a high priority on modernizing its submarine fleet, though this continues to grow modestly as it works to mature the force, integrate new technologies and expand its shipyards, according to the Pentagon’s 2023 annual report on Chinese military power.

China currently operates six nuclear-powered ballistic missile submarines, six nuclear-powered attack submarines and 48 diesel-powered attack submarines. The submarine force is expected to grow to 65 by 2025 and 80 by 2035.

That report said that the new attack submarines, as well as new surface ships and naval aircraft, “are designed to achieve maritime superiority” within the so-called first island chain, which links the Japanese islands, Taiwan and the Philippines, while also countering any moves by the U.S., Japan and others to come to the aid of Taiwan during a conflict.

U.S. Secretary of Defense Lloyd Austin has called China the Pentagon’s “pacing challenge,” and defense officials in both Washington and Tokyo say that Beijing has been heaping political and military pressure on both Japan and Taiwan, which China says is a renegade province that must be united with the mainland, by force if necessary.

In the latest example, Taiwan’s Defense Ministry said Friday morning that it had detected 41 Chinese military aircraft and six ships operating around the island in the 24 hours until 6 a.m.

Last week, Beijing sailed an aircraft carrier between two Japanese islands near Taiwan. In August, it sent a survey ship into Japanese territorial waters off Kagoshima Prefecture.

Those moves were in addition to a Chinese military spy plane’s first “incursion” into Japanese airspace. Japan called the Chinese move “utterly unacceptable” and said it considered the flight a threat to the country’s safety.