WASHINGTON — A U.S. intelligence report concludes that North Korea has advanced its nuclear know-how to the point that it could arm a ballistic missile with a nuclear warhead, a jarring revelation in the midst of bellicose threats from the unpredictable communist regime.

President Barack Obama urged calm, calling on Pyongyang to end its saber-rattling while sternly warning that he would “take all necessary steps” to protect American citizens.

The new American intelligence analysis, disclosed Thursday at a congressional hearing, says the Pentagon's intelligence wing has “moderate confidence” that North Korea has nuclear weapons capable of delivery by ballistic missiles but that the weapon was unreliable.

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Rep. Doug Lamborn, a Republican, read aloud what he said was an unclassified paragraph from a secret Defence Intelligence Agency report that was supplied to some members of Congress. The reading seemed to take Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, by surprise, who said he had not seen the report and declined to answer questions about it.

The DIA conclusion was confirmed by a senior congressional aide who spoke on condition of anonymity because the Pentagon had not officially released the contents. The aide said the report was produced in March.

Since the beginning of March, the Navy has moved two missile defence ships closer to the coast of the Korean Peninsula, in part to protect against a potential missile launch aimed at Guam, a U.S. territory in the Pacific. The Pentagon also has announced it will place a more advanced land-based missile defence on Guam, and Hagel said in March that he approved installing 14 additional missile interceptors in Alaska to bolster a portion of the missile defence network that is designed to protect all of U.S. territory.

On Thursday, the Pentagon said it had moved a sea-based X-band radar — designed to track warheads in flight — into position in the Pacific.

Notably absent from that unclassified segment of the report was any reference to what the DIA believes is the range of a missile North Korea could arm with a nuclear warhead. Much of its missile arsenal is capable of reaching South Korea and Japan, but Kim has threatened to attack the United States as well.

At the House Armed Services Committee hearing in which he disclosed the DIA assessment, Lamborn asked Dempsey, whether he agreed with it. Dempsey said he had not seen the report.

“You said it's not publicly released, so I choose not to comment on it,” Dempsey said.

But David Wright, a nuclear weapons expert at the Union of Concerned Scientists, said the DIA assessment probably does not change the views of those who closely follow developments in North Korea's pursuit of a nuclear weapon.

“People are starting to believe North Korea very likely has the capability to build a nuclear weapon small enough to put on some of their shorter-range missiles,” Wright said. “Once you start talking about warheads small enough and technically capable to be on a long-range missile, I think it's much more an open question.”

The DIA assessment is not out of line with comments Dempsey made Wednesday when he was asked at a Pentagon news conference whether North Korea was capable of pairing a nuclear warhead to a ballistic missile that could reach Japan or beyond.

In response, Dempsey said the extent of North Korean progress on designing a nuclear weapon small enough to operate as a missile warhead was a classified matter. But he did not rule out that the North has achieved the capability revealed in the DIA report.

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“They have conducted two nuclear tests,” Dempsey told a Pentagon news conference. “They have conducted several successful ballistic missile launches. And in the absence of concrete evidence to the contrary, we have to assume the worst case, and that's why we're postured as we are today.” He was referring to recent moves by the U.S. to increase its missile defence capabilities in the Pacific.

At the same House hearing where Lamborn disclosed the DIA conclusion, Defence Secretary Chuck Hagel was asked a different version of the same question: Does North Korea have the capability to strike U.S. territory with a nuclear weapon? Hagel said the answer is no.

“Now does that mean that they won't have it or they can't have it or they're not working on it?” Hagel added. “No. That's why this is a very dangerous situation.”

“Now is the time for North Korea to end the belligerent approach they have taken and to try to lower temperatures,” Obama said in his first public comments since Pyongyang threatened the United States and its allies in East Asia with nuclear attack.

Obama, speaking from the White House, said he preferred to see the tensions on the peninsula resolved through diplomatic means, but added that “the United States will take all necessary steps to protect its people.”

The North on Thursday delivered a fresh round of war rhetoric with claims it has “powerful striking means” on standby, the latest in a torrent of warlike threats seen by outsiders as an effort to scare and pressure South Korea and the U.S. into changing their North Korea policies.

Lamborn is a member of the Strategic Forces subcommittee of the Armed Services panel, which oversees ballistic missiles. A former state legislator who was elected to the House in 2006, was a member of the Tea Party caucus and belongs to the Republican Study Committee, the caucus of House conservatives

At a separate hearing Thursday, U.S. officials offered their assessment of the North Korean leader, who is a grandson of the country's founder, Kim Il Sung.

Director of National Intelligence James Clapper told the House Intelligence Committee that he thinks Kim, who took control after his father, Kim Jong Il, died in 2011, is trying to show the U.S., the world and his own people that he is “firmly in control in North Korea,” while attempting to manoeuver the international community into concessions in future negotiations.

“I don't think ... he has much of an endgame other than to somehow elicit recognition” and to turn the nuclear threat into “negotiation and to accommodation and presumably for aid,” Clapper said.

Clapper said that the intelligence community believes the North would use nuclear weapons only to preserve the Kim regime but that analysts do not know how the regime defines that.

Secretary of State John Kerry was headed Thursday to East Asia, where he planned talks with officials in Seoul, Beijing and Tokyo about North Korea.

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