* Top U.S. intel official casts doubt on NKorea capability
* U.S. seeks diplomacy, but would take "all necessary steps"
* Analyst calls report "cautious, worst-case assessment"
A Pentagon spy agency report concluded for the first time that North Korea likely has a nuclear bomb that can be launched on a missile, but U.S. defense and intelligence officials cast doubt on Pyongyang's atomic weapons capabilities.
Illustrating the high stakes surrounding the escalating tensions on the Korean peninsula, a study by the Pentagon's Defense Intelligence Agency stoked fears that North Korea could be closer to being able to launch a nuclear missile.
The secret assessment, mistakenly marked as unclassified and partially revealed at a congressional hearing on Thursday, said the agency had "moderate confidence" that North Korea possessed nuclear weapons that could be fitted onto ballistic missiles. But it said any such missile would probably be unreliable.
The evaluation, dated last month, was made public by Republican Representative Doug Lamborn as he questioned senior Pentagon officials about North Korea's nuclear weapons program during a hearing of the House of Representatives Armed Services Committee.
"DIA assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles, however the reliability will be low," said Lamborn. He was quoting from a DIA report entitled "Dynamic Threat Assessment 8099: North Korea Nuclear Weapons Program (March 2013)."
U.S. officials and South Korea sought to play down the DIA evaluation.
Pentagon spokesman George Little said, "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."
James Clapper, the country's senior intelligence official, warned that the assessment was not necessarily shared by the wider U.S. intelligence community.
"I would add that the statement read by the Member is not an Intelligence Community assessment. Moreover, North Korea has not yet demonstrated the full range of capabilities necessary for a nuclear armed missile," Clapper, the director of national intelligence, said in a statement.
But the release of part of the DIA report will likely raise tension on the Korean peninsula, where North Korea has stationed as many as five medium-range missiles on its east coast, according to assessments by Washington and Seoul, possibly in readiness for a test-launch that would demonstrate its ability to hit U.S. bases on Guam.
"This is the first ... case where the U.S. intelligence community positively concludes ... that North Korea apparently has some rudimentary weapons capability or warhead capability for ballistic missiles," said Hans Kristensen, a nuclear weapons analyst at the Federation of American Scientists.
General Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, declined to comment as he had not seen the DIA report, suggesting it was not a major part of the military's thinking as it beefed up anti-missile defenses around the Pacific in the face of threats of war from North Korea in recent weeks.
South Korea's Defense Ministry said it did not believe North Korea had succeeded in readying a nuclear warhead for a missile, a process known as "miniaturizing."
Lamborn did not say what range any nuclear-capable North Korean missiles might have. Kristensen said one analyst recently claimed nuclear warhead capability for North Korea's Nodong short- to medium-range missile. It would be able to hit U.S.-based facilities in the region, including South Korea and probably Japan.
The United States and South Korea have plans to respond proportionately to North Korean provocations like the shelling of an island or attacking a ship.
But not wanting to increase the tension in Korea, Washington has not been explicit about how it would respond to an incident involving nuclear arms, beyond saying it was capable of defending itself and its partners.
President Barack Obama said on Thursday the United States would work diplomatically to reduce tensions with North Korea, but warned that Washington would take "all necessary steps" to protect America and its allies.
The United States has revamped its missile defense plans and positioned two guided-missile destroyers in the Western Pacific recently.
In the latest move, the Pentagon is to deploy a Terminal High Altitude Area Defense system to Guam in the coming weeks after adding 14 new anti-missile interceptors in Alaska.
The manner of the release of the report raised questions about how a classified document that was apparently just a tentative assessment could become public knowledge and set off further worries about North Korea.
A U.S. official said Lamborn had done nothing wrong in releasing the statement in Congress. He said the quotation cited by the congressman was in a section of the report that had been erroneously marked unclassified.
The Defense Intelligence Agency that gathers information about the capacity and strategic intentions of foreign militaries. It was criticized after the start of the Iraq war in 2003 for being too bullish in predicting that Baghdad might have weapons of mass destruction.
The consensus inside the U.S. government is that North Korea does not yet have a nuclear device that would fit longer-range missiles that conceivably could reach U.S. territories.
"It's very clear that it cannot at this stage include long-range ballistic missiles because they're just basically not developed sufficiently yet to be able to do this," Kristensen said.
Most observers say Pyongyang has no intention of starting a war that would likely bring its own destruction, but they warn of the risks of miscalculation.
Greg Thielmann, a former State Department intelligence analyst now with the Arms Control Association advocacy group, said that while he did not have access to the classified material cited in Congress, what was said publicly about DIA's assessment sounded quite tentative.
"It really says to me that this is a speculative statement," Thielmann said. "Moderate (confidence) is higher than low confidence but it doesn't say they know very much."
He described the DIA statement as a "cautious worst-case assessment."