(CNN) -- The Pentagon's intelligence arm has assessed with "moderate confidence" that North Korea has the ability to deliver a nuclear weapon with a ballistic missile, though the reliability is believed to be "low."
Disclosed first by a congressman at a hearing on Thursday and then confirmed to CNN by the Defense Department, the assessment by the Defense Intelligence Agency is the clearest acknowledgment yet by the United States about potential advances in North Korea's nuclear program.
The surprise development comes amid heightened tensions on the Korean Peninsula.
North Korea has unleashed a torrent of dramatic threats against the United States and South Korea in recent weeks, including that of a possible nuclear strike.
The Obama administration calculates a test launch of mobile ballistic missiles could come at any time. But a senior administration official said there is no indication that missiles believed being readied for tests have been armed with any nuclear material.
Pentagon spokesman George Little said later Thursday, "It would be inaccurate to suggest that the North Korean regime has fully tested, developed, or demonstrated the kinds of nuclear capabilities referenced in the passage" of the DIA study.
Rep. Doug Lamborn, R-Colorado, read from an declassified version of the intelligence assessment at a House Armed Services Committee hearing.
"DIA assess with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles, however, the reliability will be low."
Reliability is apparently a reference to the accuracy of the missiles.
Joint Chiefs Chairman Gen. Martin Dempsey, who was testifying before the House committee, appeared to be caught off guard when asked by Lamborn whether he agreed with the DIA assessment.
"Well, I haven't seen it," Dempsey replied. "And you said it's not publicly released, so I -- I choose not to comment on it."
It is was not clear whether other U.S. intelligence agencies, such as the CIA, agree with the defense analysis.
The U.S. intelligence about North Korea revealed Thursday was "mistakenly" marked as declassified, according to an administration and a defense source.
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But Lamborn told CNN's "AC360" he acted properly, reciting declassified information.
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"Whether it was a mistake or not, I can't answer that," Lamborn said of the information release. "Given the seriousness of the threat, this is something that I think people do need to know about."
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A House Armed Services Committee aide said staffers checked with the DIA to confirm the declassification was made before Lamborn read the passage.
U.N. Secretary-General Ban Ki-moon told CNN's Wolf Blitzer, "We do not have any independent information to verify" the DIA study's assessment.