Kerry backs S. Korea bid to ease crisis with North

US Secretary of State John Kerry Friday demanded North Korea scrap an expected missile test and dial down its blood-curdling rhetoric -- but also backed new peace overtures by Seoul and appealed for reason to prevail.

With a US intelligence report offering a chilling new evaluation of Pyongyang's offensive ability as it aimed nuclear threats at Japan, Kerry urged China to step in and said the North would never be accepted as a nuclear power.

Visiting Seoul to give fulsome US backing to military ally South Korea, Kerry mixed tough talk with more conciliatory comments about the prospects for a peaceful way out of a crisis that has sent inter-Korea tensions soaring.

In particular, he said Washington chose to "honour" the vision of South Korea's new President Park Geun-Hye, who was elected on a pledge of greater engagement with Pyongyang.

"We're prepared to work with conviction that relations between North and South can improve and they can improve very quickly," Kerry said.

"I think we have lowered our rhetoric significantly and we are attempting to find a way for reasonableness to prevail here," he added.

Park has made a series of statements in recent days hinting at a dialogue with Pyongyang.

Yonhap news agency quoted Park as telling ruling party officials Friday that the South should meet with the North and "listen to what North Korea thinks".

There was no indication what form such a meeting might take.

Many observers say the crisis that has engulfed the region since North Korea staged a rocket launch and atomic test was manufactured by Pyongyang to try to force Washington into agreeing to direct talks.

Kerry made clear that a US-North dialogue was not currently on the table and stressed that any talks by any parties would require a change of course and tone from Pyongyang.

"The rhetoric that we are hearing from North Korea is simply unacceptable by any standards," Kerry told a news conference in Seoul alongside South Korean Foreign Minister Yun Byung-Se.

His arrival coincided with a new commentary from the North's official Korean Central News Agency, which warned of Japan being "consumed in nuclear flames" should it get involved in any conflict on the peninsula.

Japan positioned Patriot missile batteries around Tokyo as a pre-emptive defensive measure after US and South Korean intelligence reports suggested North Korea was preparing an imminent mid-range missile launch.

Kerry directly warned North Korean leader Kim Jong-Un that any such launch would be a "huge mistake".

The apocalyptic threats that have been flooding out of Pyongyang for the past month and a half were given extra menace Friday by a US military spy agency report that the North had a nuclear-armed ballistic missile.

North Korea has no proven capacity to shrink a nuclear device onto a missile tip.

But the Defense Intelligence Agency (DIA) evaluation, which was downplayed by US and South Korean defence officials, gave some official US credence to Pyongyang's claim in February that it has now mastered the technology.

Kerry, who heads to Beijing on Saturday, said it was time China -- whose trade and aid have propped up North Korea since the end of the Cold War -- to intervene with its wayward ally if it wants to safeguard regional stability.

"China has an enormous ability to help make a difference here," he said.

Intelligence officials in Seoul say the North, as a show of force, has two medium-range missiles ready for imminent launch from its east coast, and South Korea and Japan are on high 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" were 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 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.