Media playback is unsupported on your device Media caption President Obama has warned N Korea to end its "belligerent approach"

The US says it is urging China to use all its leverage to help rein in North Korea's "destabilising" actions.

US Secretary of State John Kerry is in South Korea, where he is expected to call on China to evoke "a sense of urgency" in its talks with the North.

Pyongyang has ratcheted up tensions in the region, threatening nuclear strikes against South Korea and the US.

A leaked US intelligence report has said the North may now be capable of mounting nuclear warheads on a missile.

On Thursday, a US Congressman read out what he said was an unclassified section of a Defense Intelligence Agency study. He said it assessed "with moderate confidence" that the North could fire a nuclear-armed missile, though with "low reliability".

The North has tested both nuclear weapons and missiles, but it had been thought it had not yet developed a device small enough to be a viable and deliverable weapon.

Such a development would change the past 20 years of diplomacy, says the BBC's Lucy Williamson in Seoul, all of which has been aimed at stopping North Korea from getting that sort of weapon.

But the Pentagon later denied the report, with spokesman George Little saying 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".

South Korea is currently on a high state of alert amid indications that the North is preparing for a missile test.

Pyongyang has moved two Musudan ballistic missiles to its east coast. Estimates of their range vary, but some suggest the missiles could travel 4,000km (2,500 miles).

That would put US bases on Guam within range, although it is not believed that the Musudan has been tested before.

'Stake in stability'

Mr Kerry is making his first trip to Asia since becoming secretary of state. He will spend time in Seoul and Tokyo as well as in Beijing, North Korea's last remaining ally and its major trading partner.

Musudan missile The Musudan, also known as the Nodong-B or the Taepodong-X, is an intermediate-range ballistic missile. Its likely targets are Okinawa, Japan, and US bases in the Pacific

Range estimates differ dramatically. Israeli intelligence suggests 2,500km, while the US Missile Defense Agency estimates 3,200km; other sources put the upper limit at 4,000km

These differences are due in large part to the fact that the missile has never been tested publicly, according to the Center for Nonproliferation Studies. Its payload is also unknown Missile defences in the region North Korea's missile programme

A senior administration official told reporters on board Mr Kerry's plane: "It is no secret that China has most leverage, most influence, with North Korea and I think fundamentally we would want them to use some of that leverage because otherwise it is very destabilising and it threatens the whole region."

The official added that, although Washington was not privy to conversations between China and North Korea, "we would want China to bring a sense of urgency, the need to stop this escalation, into that debate".

"China has a huge stake in stability and the continued North Korean pursuit of a nuclear armed missile capability is the enemy of stability. That gives us and the Chinese a very powerful objective in common, namely denuclearisation," the official said.

US President Barack Obama has urged Pyongyang to end its "belligerent approach... and to try to lower temperatures".

But he warned that while he preferred to see tensions resolved through diplomatic means, "the United States will take all necessary steps to protect its people".

China, meanwhile, has denied reports that it is deploying troops along the North Korean border.

A defence ministry official said Beijing was "paying close attention to the development of the current situation on the Korean Peninsula and has always been committed to safeguarding peace and stability in Northeast Asia," the state Xinhua news agency reports.

Anniversary approaches

Media playback is unsupported on your device Media caption North Korea has been making threats against South Korea, Japan and US bases in the region

North Korea has increased its warlike rhetoric following fresh UN sanctions imposed after its third nuclear test in February and joint military manoeuvres by the US and South Korea.

The North says it will restart a mothballed nuclear reactor, has shut an emergency military hotline to the South and has urged countries to withdraw diplomatic staff, saying it cannot now guarantee their safety.

However, in the past few days North Korea's media appear to be in more of a holiday mood, due to the approach of Monday's celebrations marking the birth of national founder Kim Il-sung - a potential launch date for a new missile test.

On Thursday, foreign ministers from the G8 group of nations condemned in the "strongest possible terms" North Korea's nuclear weapons and ballistic missile programmes.

Following talks in London, UK Foreign Secretary William Hague said that if the North conducted another missile launch or nuclear test "we have committed ourselves to take further significant measures".