North Korea is believed to be capable of building a nuclear weapon small enough to fit atop a long-range missile and fire it at the U.S. mainland, the commander of the U.S. Northern Command said Tuesday.
  

Adm. William Gortney also said at a Pentagon briefing that the U.S. military is very concerned about the North's new road-mobile KN-08 intercontinental ballistic missile because it would be difficult to detect indications of a potential launch.
  

"Our assessment is that they have the ability to put it on -- a nuclear weapon on a KN-08 and shoot it at the homeland," Gortney told the briefing. "That's the way we think. That's our assessment of the process. We haven't seen them test the KN-08 yet and we're waiting to do that."
  

It was the latest in a series of similar assessments by U.S. military commanders.
  

Last month, Adm. Cecil Haney, commander of the U.S. Strategic Command, told a Senate Armed Services Committee hearing that he believes the North has "already miniaturized" some of its nuclear weapons.
  

In October, Gen. Curtis Scaparrotti, commander of U.S. Forces Korea, said that as USFK commander, he assumes the North has the capabilities to miniaturize nuclear warheads and that the country has "the technology to potentially actually deliver what they say they have."
  

North Korea has conducted three underground nuclear tests so far, in 2006, 2009 and 2013. The country has also conducted a series of long-range missile or rocket launches since 1998. In its latest launch in late 2012, the North succeeded in putting a satellite into orbit.
  

Experts have warned that it is only a matter of time until the North develops nuclear-tipped missiles.
  

The Northern Command, based at Peterson Air Force Base, Colo., is charged with coordinating homeland defense. (Yonhap)