The U.S. Defense Department stands by an assessment from a top military commander that North Korea is capable of building a nuclear weapon small enough to fit atop its new KN-08 intercontinental ballistic missile and firing it at the U.S. mainland, a Pentagon official said Wednesday.
  

The remark runs counter to the South Korean Defense Ministry's insistence that the assessment from Adm. William Gortney, commander of North American Aerospace Defense Command and U.S. Northern Command, is different from the U.S. government's official assessment.
  

"We stand behind Adm. Gortney's statement," a Pentagon official told Yonhap News Agency by phone.
  

Gortney made the remark during a Pentagon briefing on Tuesday, saying he believes the North has "the ability to put it on -- a nuclear weapon on a KN-08 and shoot it at the homeland."
  

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."
  

The road-mobile KN-08, though untested, is believed to have a range of at least 5,500 km, which puts Alaska at risk. U.S. officials, including Adm. Samuel Locklear, commander of the U.S. Pacific Command, have expressed concern about the missile, saying the missile is harder to keep an eye on as it can be launched from mobile launchers.
  

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)