The last of the nuclear-powered attack submarines built by China was sunk several months ago, a senior U.S. defense official said today, a development that potentially embarrasses Beijing, which is seeking to expand its military capabilities.

China already has the world’s largest naval fleet, with more than 370 ships, and has begun production of a new generation of submarines capable of carrying nuclear weapons.

The US official, who asked not to be named, said the new nuclear-powered attack submarine sank next to a pier sometime between May and June. It is not clear what led to its sinking, nor whether it was loaded with nuclear fuel.

“Apart from the obvious questions, about the training (of the crew) and the quality of the materials, the incident raises deeper concerns about the level of accountability of the PLA (Chinese military) and oversight of China’s defence industry, which has long been plagued by corruption,” the source added.

“It is not surprising that the PLA attempted to conceal” the sinking, he assessed.

China’s embassy in Washington did not respond to a request for comment. The news was first reported by the Wall Street Journal.

Satellite images from Planet Labs taken in June show cranes at the Wuchang shipyard, where the submarine was supposed to be docked.

In 2022 China had six nuclear-powered ballistic missile submarines, six nuclear-powered attack submarines and 48 diesel-powered attack submarines, according to a Pentagon report on Beijing’s military capabilities. The submarine force is expected to grow to 65 by 2025 and 80 by 2035, the US Defense Department estimates.