WASHINGTON (AP) — Satellite imagery showed that China’s newest nuclear-powered attack submarine sank alongside a pier while under construction, a senior U.S. defense official said Thursday.

China's first Zhou-class submarine sank likely 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 U.S. considers China’s rise one of its main future 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.

The sinking was first reported by The Wall Street Journal.