America's secretary of state John Kerry has arrived in South Korea, as the region braces itself for a planned missile test by North Korea.
Meanwhile, officials in Washington are playing down a leaked intelligence report, which claims North Korea may be able to make a missile armed with a nuclear warhead.
South Korea has also said it "doubts" its neighbour would be able to launch any such weapon.
However, North Korea is still threatening strikes and has moved two missiles to its east coast.
The US intelligence report says North Korea has advanced its nuclear know-how to the point that it could arm a ballistic missile with a nuclear warhead.
The new American intelligence analysis, disclosed at a hearing in Congress, 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.
Rep Doug Lamborn read aloud what he said was an unclassified paragraph from a secret Defence Intelligence Agency report supplied to some members of Congress.
The reading seemed to take General Martin Dempsey, chairman of the Joint Chiefs of Staff, by surprise. He said he had not seen the report and declined to answer questions about it.
Pentagon press secretary George Little said: “While I cannot speak to all the details of a report that is classified in its entirety, it would be inaccurate to suggest that the North Korean regime has fully tested, developed or demonstrated the kinds of nuclear capabilities referenced” in Mr Lamborn’s remarks.
“The United States continues to closely monitor the North Korean nuclear programme and calls upon North Korea to honour its international obligations,” Mr Little added.
The DIA conclusion was confirmed by a senior congressional aide who said the report was produced in March.
President Barack Obama urged calm, calling on Pyongyang to end its sabre-rattling while warning that he would “take all necessary steps” to protect American citizens.
Since the beginning of March, the US 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 US territory in the Pacific.
The Pentagon also has announced it will place a more advanced land-based missile defence on Guam, and defence secretary Chuck Hagel said he approved 14 additional missile interceptors in Alaska to bolster a portion of the missile defence network designed to protect all US territory.
Yesterday 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 Jong Un has threatened to attack the United States as well.
At the House Armed Services Committee hearing in which he revealed the DIA assessment, Mr Lamborn asked Gen Dempsey, whether he agreed with it. Gen Dempsey said he had not seen the report.
“You said it’s not publicly released, so I choose not to comment on it,” he said.
But David Wright, a nuclear weapons expert at the Union of Concerned Scientists, said the DIA assessment probably did not change the views of those who closely followed 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,” he 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.”