The United States berated North Korea's belligerence and pressed China to rein in its ally, as US officials downplayed a chilling spy agency report that Pyongyang has a nuclear-armed missile.
The alarming assessment of the North's potential to unleash havoc came just prior to US Secretary of State John Kerry's arrival in Seoul on Friday, as regional tensions mounted over an expected missile launch by Pyongyang.
A senior official travelling with Kerry again pressed China to encourage its wayward ally North Korea to stop its destabilising nuclear activities and threats to the region.
The North's December rocket launch and February nuclear test, and its fury over subsequent UN sanctions, are at the core of a crisis that has seen Pyongyang threaten nuclear strikes against the United States and South Korea.
Kerry was to be briefed first-hand on the tensions from top US military commanders on the ground, ahead of meetings with new South Korean President Park Geun-Hye and Foreign Minister Yun Byung-Se.
President Barack Obama said "nobody wants to see a conflict", but emphasised that the United States was ready to take "all necessary steps to protect its people" and defend its allies in the region.
"We both agree that now is the time for North Korea to end the kind of belligerent approach that they've been taking," Obama said after White House talks with UN Secretary General Ban Ki-moon.
"It's important for North Korea, like every other country in the world, to observe basic rules and norms."
The top US official travelling with Kerry, who will also visit Beijing and Tokyo during his trip to Asia, said China had a key role to play in the crisis.
"China has a huge stake in stability, and the continued North Korean pursuit of a nuclear-armed missile capability is the enemy of stability," the official said.
In Washington, Congressman Doug Lamborn, reading from an unclassified portion of a Defense Intelligence Agency report, said Pyongyang could be capable of launching a nuclear warhead, albeit an unpredictable one.
"DIA assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles," said the report, according to the Republican lawmaker. "However, the reliability will be low."
But the Pentagon and the director of national intelligence quickly threw cold water on the assessment.
Pentagon spokesman George Little said it would be "inaccurate" to suggest North Korea had demonstrated the capabilities referenced by Lamborn -- a remark echoed by Director of National Intelligence James Clapper.
The leaked intelligence marked the first time the US government has suggested North Korea may have succeeded in miniaturizing a nuclear device -- a potentially game-changing scenario for the strategic balance in East Asia.
South Korea was sceptical, with Defence Ministry spokesman Kim Min-Seok saying it was "still doubtful" that the North had produced a warhead small enough to fit on a missile.
Intelligence officials in Seoul say the North has two mid-range missiles ready for imminent launch from its east coast, and South Korea and Japan remained on heightened alert for any test.
Pyongyang has not officially announced plans for a launch, but a state body in charge of inter-Korean exchanges stressed Thursday that "powerful strike means" had been put in place.
Observers believe a launch is most likely in the build-up to Monday's anniversary of the birth of late founder Kim Il-Sung, for which celebrations are already well under way in Pyongyang.
The mid-range missiles mobilised by the North are reported to be untested Musudan models with an estimated range of up to 2,485 miles (4,000 kilometres).
That would cover any target in South Korea and Japan, and possibly even US military bases on the Pacific island of Guam.
Yonhap news agency quoted military sources as saying the North was moving multiple missiles around in an apparent bid to confuse outside intelligence gatherers about its intentions.