The United States has not seen North Korea demonstrate its nuclear warhead miniaturization capabilities, but takes threats from Pyongyang seriously and is prepared for the worst, officials said Wednesday.

Pentagon officials made the remarks after North Korea released photos of what it claims was a miniaturized nuclear warhead, with leader Kim Jong-un claiming that the country succeeded in making nuclear bombs small enough to fit on ballistic missiles.

"We are aware of the report out of North Korea. The U.S. government assessment has not changed," Cdr. Bill Urban, a Pentagon spokesman, said in response to a query from Yonhap News Agency about the North's claims.

"We have not seen North Korea test or demonstrate the ability to miniaturize a nuclear weapon and put it on an ICBM. Given the consequences of getting it wrong, it is prudent for a military planner to plan for the worst," he said.

North Korea poses a range of ballistic missile threats, and the U.S. maintains a range of missile defense capabilities to counter those threat, Urban said, adding that the U.S. is confident in its ability to counter a ballistic missile threat such as that posed by the North.

Pentagon spokesman Capt. Jeff Davis also stressed the importance of preparing for the worst-case scenario.

"Even though they haven't tested it per se, we know that they've done nuclear tests, we know that they say they've miniaturized, so we at some level have to take them at their word and that's why we have capabilities with our ballistic missile defense systems to be able to meet what we consider to be a limited untested threat," he told reporters at a Pentagon briefing.

Davis noted that the North has conducted a couple of successful launches into space that used three-stage rockets, the same technology and components as an ICBM.

"They've got this untested capability in terms of end-to-end testing, but we have to be able to be ahead of that threat, to outpace that threat. That's why we have our ballistic missile systems that we do to protect the homeland," he said. "That's why we work with Japan and the Republic of Korea, our allies in the region, to be able to ensure that they have the ability to defend against attacks with shorter-range missiles. And it's absolutely something that we need to take seriously even if it's untested."

During a Pentagon briefing earlier this week, U.S. Air Force Chief of Staff Gen. Mark Welsh said that he does not believe the North has reached the stage where it can "combine a long-range missile with a warhead that was operable." 

State Department spokesman John Kirby said he takes the North's claims seriously.

"We've seen the comments and as before, we take these comments seriously. We have to," Kirby said. "This young man has proven that he's perfectly willing and able to flout and to violate his international obligations. Of course, we take it seriously." 

"I'm not going to get into an intelligence analysis of this specific capability that he alluded to," he said of the North's nuclear warhead miniaturization capabilities.

Noting Pyongyang has made a series of belligerent statements, Kirby urged the North's leader "to pay more attention to the North Korean people and taking care of them than in pursuing these sorts of reckless capabilities."

Kirby called the North's leader as "young man" and "young dictator." 

Asked if he uses the words to belittle him, Kirby said, "No. He belittles himself with his own conduct and the way he treats his own people." (Yonhap)