China has fielded new, longer-range ballistic missiles on its six nuclear-powered submarines, allowing it to hit the continental U.S. from much closer to its own shores, the U.S. acknowledged publicly for the first time Friday.

China’s six Jin-class submarines are now "equipped with JL-3 intercontinental ballistic missiles,” Adm. Sam Paparo, the head of the U.S. Pacific Fleet, told military reporters at a conference in Washington. "They were built to threaten the United States.”

"We keep close track of those submarines,” Paparo said.

In its annual report on China’s military a year ago, the Pentagon said the People’s Liberation Army Navy would eventually gain the ability to target the U.S. from its own coastal waters. Paparo declined to say if any of the six submarines have conducted so-called deterrence patrols close to Hawaii.

The Congressional Research Service estimates that the previous model submarine-launched ballistic missile, the JL-2, had a range of about 7,200 kilometers (4,464 miles), giving Chinese submarines the ability to attack parts of Alaska from areas close to China. But the subs would have had to enter waters east of Hawaii to hit the U.S. east coast.

The longer-range JL-3 changes that equation. It would allow China to target the continental U.S. "from a protected bastion within the South China Sea,” U.S. Strategic Command Commander Adm. Charles Richard told the Senate Armed Services Committee back in March.

Some news reports have suggested that the JL-3 could have a range of more than 10,000 km (6,200 miles).

"They’re now capable of continuous at sea determined patrols with their Jin-class submarines” and "more are coming,” Richard said. "They have a true nuclear command and control system.”