US Secretary of State John Kerry Friday demanded North Korea abandon an expected missile launch as Pyongyang turned its nuclear threats on Japan amid a chilling new evaluation of its offensive capability.

Kerry, visiting Seoul to give fulsome US backing to military ally South Korea, joined President Barack Obama in decrying North Korea's incendiary rhetoric -- and urged China to step in.

The air of crisis that has engulfed the region for weeks, since North Korea staged a rocket launch and atomic test, was given even greater menace from a US intelligence report that said it may now have a nuclear warhead in its arsenal.

US and South Korean military officials downplayed the assessment by the Defense Intelligence Agency (DIA), but Pyongyang warned of the direst results if Japan executes its threat to shoot down any North Korean missile.

The North's Korean Central News Agency (KCNA) said such a "provocative" action would end with its instigators "consumed in nuclear flames".

"Japan is always in the cross-hairs of our revolutionary army and if Japan makes a slightest move, the spark of war will touch Japan first," KCNA said in a commentary.

Japan positioned Patriot anti-missile batteries around Tokyo this week to protect the 30 million people who live in the vast conurbation.

"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.

Any missile launch would be a "huge mistake", he said, adding that the US and the rest of the international community would never accept North Korea as a nuclear state.

But with military tensions on the Korean peninsula at their highest level for years, Kerry said Washington also 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 her 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.

Kerry also said it was high time for 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 truly wants to safeguard regional stability.

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

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

Obama said earlier that "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.

North Korea has no proven capacity to shrink a nuclear device onto a missile tip. But for the first time, the DIA evaluation gave official US credence to Pyongyang's claim in February that it has now mastered the technology.

"DIA assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles," said the report cited by a Republican lawmaker at a congressional hearing. "However, the reliability will be low."

But Pentagon spokesman George Little said it would be "inaccurate" to suggest North Korea had shown that it has such expertise, in a remark echoed by Director of National Intelligence James Clapper.

South Korea was also sceptical. Defence Ministry spokesman Kim Min-Seok said it was "still doubtful" that North Korea had perfected the deep technical expertise needed to craft a nuclear missile warhead.

Facts about N. Korea's nuclear weapons capability
Seoul (AFP) April 12, 2013 - Findings from a report by the US military spy agency suggest North Korea could be capable of launching a ballistic missile with a nuclear warhead.

The Pentagon and South Korea's defence ministry have cast strong doubts on the assessment which adds to the general uncertainty surrounding Pyongyang's secretive nuclear weapons programme.

WHAT DO WE KNOW FOR SURE?

Not a great deal.

North Korea has conducted three nuclear tests in 2006, 2009 and 2013.

The 2013 underground test was by far the most powerful. But the radioactive fallout was well contained, frustrating intense efforts by US, South Korean and Japanese monitors to learn more about the nature of the detonation.

Numerous experts believe it may have been a uranium bomb, rather than a plutonium device as used in the previous two tests.

North Korea has only successfully tested one medium-range ballistic missile, the Rodong-1, with a range of 1,300km (800 miles).

In 1998 it launched a Taepodong-1 (2,500km) over Japan, but the third stage apparently exploded. A Taepodong-2 (6,700km) was tested in 2006 but blew up after 40 seconds.

In December, the North successfully put a satellite into orbit with its Unha-3 long-range rocket.

WHAT DOES NORTH KOREA CLAIM?

A great deal.

The North Korean military says it already has an inter-continental ballistic missile (ICBM) capability and can accurately deliver a nuclear warhead to targets as far away as the continental United States.

Pyongyang said the February 2013 test was of a "miniaturised" device and added that it now had a "diversified" weapons capability -- suggesting both uranium and plutonium bombs.

WHAT IS THE EXPERT CONSENSUS?

Mixed.

Nearly all experts agree that, despite the Unha-3 launch, North Korea is years from developing a genuine ICBM capability.

The issue of miniaturisation is more debated. A few experts believe Pyongyang may have already mastered this technique, but the majority believe it would require one, or maybe two more nuclear tests to develop a working warhead that could fit on a missile.

Delivery system capability is highly speculative. Even at the mid-range level, the North's ballistic missile record is extremely shaky.

The Unha-3 launch demonstrated its mastery off propulsion issues, but there are still large questions regarding accuracy and reliability.