The incident involving the first of the Zhou-class line of vessels reportedly occurred in Wuhan in late May or early June

China’s newest nuclear submarine sank while in dock several months ago, US media outlets claimed on Thursday, citing sources. Beijing has reportedly avoided making the incident public amid a major effort to expand its navy.

Unnamed US officials told the Wall Street Journal and CNN that the incident took place at a shipyard near Wuhan in late May or early June. According to Reuters, satellite images from June appear to show cranes at the facility in the Wuchang district where the submarine would have been docked, indicating a potential salvage operation.

CNN described the vessel as an attack submarine, the first of the new Zhou class, which has a distinctive X-shaped stern that increases its maneuverability. The WSJ said the submarine was completing its outfitting before leaving port when it went down.

A senior defense official told CNN that it was unclear whether the sub had nuclear fuel on board when it sank or whether it was emptied after the incident. There is no indication of what caused the sinking, or whether Chinese authorities have tested the water for contamination.

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US officials interviewed by the WSJ suggested that Beijing was trying to cover up the controversy because it never reported the incident. “It’s not surprising that the PLA Navy would try to conceal the fact that their new first-in-class nuclear-powered attack submarine sank pierside,” the paper’s source said. This raises questions not only about China’s training standards but also its accountability and oversight of the defense industry, the source alleged.

A spokesperson for the Chinese embassy in the US neither confirmed nor denied the incident. “We are not familiar with the situation you mentioned and currently have no information to provide,” he said.

According to a 2021 Pentagon report, China has the world’s largest navy in terms of numbers, with a total of about 355 ships and submarines. In 2022, the US estimated that Beijing had six nuclear-powered ballistic missile submarines, six nuclear-powered attack submarines, and 48 diesel-powered attack submarines. Beijing plans to dramatically expand its navy amid tensions with the US over Taiwan and a number of regional powers in the South China Sea.