Secretary of State John Kerry dismissed as "unacceptable by any standard" weeks of bellicose warnings of impending nuclear war by North Korea and said Washington would never accept the reclusive state becoming a nuclear power.
Kerry, addressing reporters after talks with South Korean President Park Geun-hye and leaders of the 28,000-strong U.S. military contingent in the country, also said the United States would defend its allies in the region if necessary.
"The rhetoric we are hearing from North Korea is simply unacceptable by any standard," Kerry said. "We are all united in the fact that North Korea will not be accepted as a nuclear power."
North Korea has issued weeks of shrill warnings to the United States and South Korea, including of waging thermonuclear war, after the imposition of new U.N. sanctions in response to its third nuclear arms test in February.
Kerry's visit coincides with preparations for the anniversary Monday of North Korean state founder Kim Il-sung's birth, a possible pretext for a military show of strength.
Speculation has mounted that Pyongyang may launch a medium-range missile after reports in South Korea and the United States that missiles had been moved into suitable locations.
Kerry said North Korean leader Kim Jong-un would be making a "huge mistake" if he proceeded with a launch.
He also said that China, the North's sole diplomatic and financial ally, has the ability to make a difference on influencing its policies.
Hours before his arrival, a U.S. government agency said North Korea had a nuclear weapon it can mount on a missile, adding an ominous dimension to discussions in Seoul.
However, the assessment by the Pentagon's Defense Intelligence Agency 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 30-year-old 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."
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 Rep. 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.
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.
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 criticized after the start of the Iraq war in 2003 for being too bullish in predicting Baghdad might have weapons of mass destruction.
Its conclusion about North Korea follows more than a month of rising tension on the Korean peninsula.
Asked about 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 denuclearization via talks and consultations. No matter what changes there are in the situation, we will uphold this direction."
U.N. sanctions sparked 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 2,100 miles or more would put Japan within striking distance and may threaten Guam, home to U.S. military bases.