Satellite imagery showed that China’s newest nuclear-powered attack submarine has sunk alongside a pier while under construction, a senior US defence official has said.

China’s first Zhou-class submarine likely sank between May and June, when satellite images showed cranes that would be necessary to lift it off the bottom of the river, said the official, who spoke on the condition of anonymity to provide details about the submarine loss.

China has been building up its naval fleet at a breakneck pace and the US considers China’s rise one of its main security concerns.

Beijing has not acknowledged the sinking, and the official said it was “not surprising” that China’s navy would conceal it.

The submarine’s current status is unknown.

Satellite images from Planet Labs PBC show what appears to be a submarine docked at the Shuangliu shipyard on the Yangtze River before the incident.

An image taken on 15 June appears to show the submarine either fully or partially submerged just under the river’s surface, with rescue equipment and cranes surrounding it. Booms surround it to prevent any oil or other leaks from the vessel.

A satellite image taken on 25 August shows a submarine back at the same dock as the submerged vessel. It’s not clear if it was the same one.

It remains unclear if the affected submarine had been loaded with nuclear fuel or if its reactor was operating at the time of the incident. However, there has been no reported release of radiation in the area in the time since.

As of last year, China operated six nuclear-powered ballistic missile submarines, six nuclear-powered attack submarines and 48 diesel-powered attack submarines, according to a US military report.