The United States has no evidence yet that North Korea has mastered the technology to make nuclear warheads small enough to fit atop ballistic missiles capable of reaching the U.S., a Defense Department official said Thursday.

South Korea's Defense Ministry said in its defense "white paper" report published earlier this week that it believes the communist nation has reached a "significant" point in efforts to mastering the warhead miniaturization technology.

"Gen. Scaparrotti stood in our briefing room several months ago and spoke, I think, very eloquently about this," Col. Steve Warren, a Pentagon spokesman, said, referring to the commander of U.S. Forces Korea, Gen. Curtis Scaparrotti.

"He said that it is prudent for him as a commander to prepare for such a contingency, but that we have no evidence yet that they have achieve that level of technology," Warren said in response to a Yonhap News Agency question about the U.S. assessment of the North's nuclear capabilities.

Scaparrotti made the remark in October, saying as USFK commander, he assumes that 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."

"Given their technological capabilities, that the time that they've been working on this, that they probably have the capabilities to put this together. I don't believe that they have. I don't know that they have at this point," the commander said at the time.

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 aboard a long-range rocket.

Experts have warned that it is only a matter of time for the North to develop nuclear-tipped missiles. (Yonhap)