China is for the first time keeping at least one nuclear-armed ballistic missile submarine constantly at sea, according to a Pentagon report — adding pressure on the United States and its allies as they try to counter Beijing's growing military.

The assessment of China's military said China's fleet of six Jin-class ballistic missile submarines were operating "near-continuous" patrols from Hainan Island into the South China Sea. Equipped with a new, longer-range ballistic missile, they can hit the continental United States, analysts say.

The note in the 174-page report drew little attention when it was released in late November, but shows crucial improvements in Chinese capabilities, according to four regional military attaches familiar with naval operations and five other security analysts.