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 existence of the assessment 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, General Martin E. Dempsey. Dempsey declined to comment on the assessment because of classification issues.
But on Thursday evening, the Pentagon press secretary, George Little, issued a statement that sought to qualify the conclusion of the Defense Intelligence Agency, which has primary responsibility for monitoring the missile capabilities of adversary nations but which a decade ago was among those that argued most vociferously — and incorrectly — that Iraq had nuclear weapons.
‘‘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,’’ Little said. ‘‘The United States continues to closely monitor the North Korean nuclear program and calls upon North Korea to honor its international obligations.’’
Nonetheless, outside experts said that the report’s conclusions could help explain why Hagel has said in recent weeks that the Pentagon was bolstering long-range antimissile defenses in Alaska and California, designed to protect the West Coast, and rushing another antimissile system, originally not intended for deployment until 2015, to Guam.
The disclosure of the Defense Intelligence Agency’s assessment came on the same day that the director of national intelligence, James R. Clapper, sought to tamp down fears that North Korean rhetoric could lead to an armed clash with the United States, South Korea, and regional allies, and that a high South Korean official called for dialogue with North Korea.
Clapper told a hearing of the House Intelligence Committee that, in his experience, two other confrontations with the North — the seizure of the US research ship Pueblo in 1968 and the death of two US soldiers in a tree-cutting episode in a border area in 1976 — stoked much greater tensions between the two countries. The statement by the South Korean official, Unification Minister Ryoo Kihl Jae, was televised and represented a considerable softening in tone by President Park Geun Hye’s government.
Secretary of State John Kerry, meanwhile, was scheduled to arrive in Seoul on Friday and to travel to China and Japan after that. He has two principal goals on the last leg of a six-nation trip: to encourage China to use its influence to persuade North Korea to abandon its nuclear weapons program and to reassure South Korea and Japan that the US remains committed to their defense.
North Korea has conducted three nuclear tests, including one this year, and shot a ballistic missile as far as the Philippines in December. US and South Korean intelligence agencies believe that another test — perhaps of a midrange missile called the Musadan that can reach Japan, South Korea, and almost as far as Guam — may be conducted in the coming days, to celebrate the birth of Kim Il Sung, the country’s founder.
At the Pentagon, there is particular concern about another missile, yet untested, called the KN-08, which may have significantly longer range.
“North Korea has already demonstrated capabilities that threaten the United States and the security environment in East Asia,’’ Clapper told the House Intelligence Committee.