A Chinese nuclear-powered attack submarine sank in its dock earlier this year while under construction, in a major setback for the Chinese military, according to US defence officials. Speaking on condition of anonymity to the BBC's US partner, CBS, the officials said the loss of the submarine probably happened between May and June.

A U.S. officials says China's newest nuclear-powered submarine in its fleet sank in spring while docked at a pier but Chinese government officials have taken steps to cover it up.

Building a potent nuclear submarine force would help China tighten control over its peripheral waters and improve its ability to seize Taiwan.