WASHINGTON — A U.S. intelligence agency has concluded that North Korea has the capability to develop nuclear warheads small enough to fit on a ballistic missile, a congressman disclosed Thursday.

WASHINGTON — A U.S. intelligence agency has concluded that North Korea has the capability to develop nuclear warheads small enough to fit on a ballistic missile, a congressman disclosed Thursday.

Although U.S. experts believe North Korea cannot hit the U.S. mainland with its missiles, a significant improvement in North Korea's weapons technology would be deeply disconcerting for U.S. policymakers. It also would help explain the United States' measures — including an emphasis on its ability to respond with nuclear weapons — after weeks of warlike rhetoric from North Korea.

Regional intelligence officials and analysts say North Korea is poised to launch as many as five missiles from its east coast, but that the planned launches did not appear to be in preparation for war. They said the launches likely were to be only part of a military exercise and would not pose a threat to the U.S. or its allies, Japan and South Korea.

Analysts said the exercise probably would be part of festivities to mark the birthday Monday of the country's late founder, Kim Il Sung, grandfather of the present leader.

At a House Armed Services Committee hearing focused on the budget, Rep. Doug Lamborn, R-Colo., read from what he said was an unclassified portion of a classified Defense Intelligence Agency study. "DIA assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles. However, the reliability will be low," it says.

Lamborn said the DIA study was completed last month, but the conclusion had not been made public.

Pentagon spokesman George Little said he could not address the details of a classified report. However, "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."

The DIA tends to be more aggressive in its intelligence assessments than the CIA, according to Bruce Klingner, a former CIA analyst on North Korea. He said the DIA has hinted before that North Korea could mount warheads on its missiles. It's unclear whether other U.S. intelligence agencies share the assessment.

The Pentagon said last month that it planned to augment missile defense systems in Alaska in response to the threat.

The Defense Department also said it would deploy another antimissile system to Guam, which is within range of North Korean missiles.

The Pentagon also sent two long-range B-2 bombers from their base in Missouri last month on a round-trip flight to South Korea as part of a military exercise.

North Korea said after its most recent nuclear test in February that its objective was to develop a smaller and lighter warhead with "diverse materials."

Analysts said the statement indicated North Korea was stepping up its development of a miniaturized warhead that could fit atop one of its long-range missiles, and that the device used enriched uranium as a trigger, rather than the plutonium previously used.

North Korea's official news agency said the nuclear test was successful. Seismic monitoring systems measured a resulting earthquake of magnitude 5.1, slightly higher than in previous tests. But U.S. officials said yield estimates were uncertain.

At a Pentagon news conference last month to announce expansion of the U.S. missile defense systems, Defense Secretary Chuck Hagel did not answer directly when asked when North Korea would have intercontinental ballistic missiles armed with nuclear warheads.

David Albright, a nuclear weapons expert with the Institute for Science and International Security, said in February that he believed North Korea has had the capability to mount a plutonium-based warhead on its shorter-range Rodong missiles "for some time." and was making progress on a warhead for an intercontinental ballistic missile.