WASHINGTON — A new assessment by the Pentagon’s intelligence arm has concluded for the first time, with “moderate confidence,” that North Korea has learned how to make a nuclear weapon small enough to be delivered by a ballistic missile.

The assessment by the Defense Intelligence Agency, which has been distributed to senior administration officials and members of Congress, cautions that the weapon’s “reliability will be low,” apparently a reference to the North’s difficulty in developing accurate missiles or, perhaps, to the huge technical challenges of designing a warhead that can survive the rigors of flight and detonate on a specific target.

The assessment’s existence was disclosed Thursday by Representative Doug Lamborn, Republican of Colorado, three hours into a budget hearing of the House Armed Services Committee with Defense Secretary Chuck Hagel and the chairman of the Joint Chiefs of Staff, Gen. Martin E. Dempsey. General Dempsey declined to comment on the assessment because of classification issues.

But late Thursday, the director of national intelligence, James R. Clapper Jr., released a statement saying that the assessment did not represent a consensus of the nation’s intelligence community and that “North Korea has not yet demonstrated the full range of capabilities necessary for a nuclear armed missile.”