China has said its first fleet of nuclear submarines has started sea patrols, in the latest sign of the confidence of the country’s military that has raised tensions in the region.

Xinhua, the official news agency, released photographs of what appeared to be Xia-class vessels – China’s first generation of nuclear-armed submarines, which are several decades old – saying they were being “declassified” for the first time. It said they would “gallop to the depths of the ocean, serving as mysterious forces igniting the sound of thunder in the deep sea”, and be an “assassin’s mace that would make adversaries tremble”.

The Chinese navy has in recent years increased in assertiveness as it has enhanced its capabilities. In June, the US revealed that Chinese warships had started patrolling its exclusive economic zone; the following month, Chinese destroyers passed through the strait between Russia and northern Japan for the first time.