A U.S. government agency said North Korea has a nuclear weapon it can mount on a missile, adding an ominous dimension to tensions on the Korean peninsula as U.S. Secretary of State John Kerry arrived for talks in Seoul on Friday.
However, the assessment by the Pentagon's Defense Intelligence Agency (DIA) was swiftly dismissed by several U.S. officials and South Korea.
Asked if war was imminent, a U.S. official in South Korea said: "Not at all".
Washington's greatest concern, the official said, was the possibility of unexpected developments linked to the inexperience of North Korea's 30-year-old leader, Kim Jong-un.
"Kim Jong-un's youth and inexperience make him very vulnerable to miscalculation. Our greatest concern is a miscalculation and where that may lead," said the official, who spoke on condition of anonymity.
"We have seen no indications of massive troop movements, or troops massing on the border, or massive exercises or anything like that that would back up any of the rhetoric that is going on."
Kerry was due to meet South Korean President Park Geun-hye and top brass from the 28,000-strong U.S. military contingent in the country.
Park, meeting officials from her ruling Saenuri Party before her talks with Kerry, struck a conciliatory note by suggesting Seoul should at least listen to what North Korea had to say.
"We have a lot of issues, including the Kaesong industrial zone," local media quoted her as saying. So should we not meet with them and ask: "Just what are you trying to do?'"
The president was referring to North Korea's closure this week of the jointly run Kaesong industrial park, with the loss of 53,000 jobs.
In Washington, Republican Representative Doug Lamborn quoted the DIA as saying it had concluded with "moderate confidence" that North Korea had developed a nuclear bomb that could be fitted on a ballistic missile, but added such a weapon would probably be unreliable.
South Korean and U.S. officials say Pyongyang appears set to test-launch a medium-range missile as a show of strength linked to the anniversary on Monday of the birth of North Korean state founder Kim Il-Sung. North Korea has conducted three nuclear tests, but it was not believed to be near weapons capability.
In Pyongyang, Rodong Sinmun, the mouthpiece of the ruling Workers' Party, said North Korea would never abandon its nuclear weapons program, made necessary by the "invariable ambition of the U.S. to militarily invade" North Korea.
"Strong is the will of the DPRK to strengthen the nuclear deterrence for self defence...," it said, referring to the country by its official name, the Democratic People's Republic of Korea. "The DPRK will hold tighter the treasured sword, nuclear weapons."
South Korea Skeptical
South Korea's Defence Ministry maintained it did not believe North Korea could mount a nuclear warhead on a missile.
Despite the DIA report, the Pentagon's spokesman and the U.S. national intelligence director both said it was "inaccurate" to infer Pyongyang had the proven ability to launch a nuclear missile.
The DIA was criticised after the start of the Iraq war in 2003 for being too bullish in predicting Baghdad might have weapons of mass destruction.
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Its conclusion about North Korea follows more than a month of rising tension on the Korean peninsula.
China, North Korea's only major diplomatic ally, denied reports that it was staging military drills along the North Korean border.
Asked about the U.S. reports that Pyongyang may have developed a nuclear weapon, Chinese Foreign Ministry spokesman Hong Lei said: "China upholds the maintenance of peace and stability on the Korean peninsula and pushing for its denuclearisation via talks and consultations. No matter what changes there are in the situation, we will uphold this direction."
North Korea, claiming the United States is planning to invade, has threatened Washington and Seoul with nuclear war, although most experts say Pyongyang has no intention of starting a conflict that would likely bring its own destruction.
North Korea Angry Over Sanctions, Drills
The United Nations sanctioned North Korea for its third nuclear test on Feb. 12, sparking a furious response from Pyongyang. The North has also called annual military drills between U.S. and South Korean forces a "hostile" act.
North Korea has stationed as many as five medium-range missiles on its east coast, according to defense assessments by Washington and Seoul. South Korean and U.S. officials believe it is preparing to launch a Musudan missile, whose range of 3,500 km (2,100 miles) or more would put Japan within striking distance and may threaten Guam, home to U.S. military bases.
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Despite North Korea's nuclear tests, there has been no hard evidence that it has developed a warhead small enough to mount on a missile and whether it can then ensure that missile re-enters the earth's atmosphere.
North Korea last tested a long-range rocket in December. It launched the rocket into space for the first time but the rocket did not successfully re-enter.
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" of the DIA report.
The conclusion of the DIA was not shared by the wider U.S. intelligence community, U.S. National Intelligence Director James Clapper said in a statement.
The strong 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 hit the U.S. mainland.