The North has moved two missiles to its east coast and South Korea is on alert.

Speaking in Seoul, Mr Kerry reconfirmed the US's commitment to protecting itself and its allies.

But he played down a US report that the North has a nuclear warhead, saying it was "inaccurate" to suggest it has "a working and tested" device.

A declassified section of a report from the Defense Intelligence Agency (DIA) report had warned there was "moderate" confidence that Pyongyang had developed the technology to mount a nuclear warhead on a missile.

North Korea will mark the birth of national founder Kim Il-sung on 15 April, a date which could be used for a missile launch.
It 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.In a joint news conference with his South Korea counterpart, Mr Kerry said that if Northern leader Kim Jong-un decided to go ahead with a launch "he will be choosing wilfully to ignore the entire international community, his own obligations which he has accepted, and it will be a provocative and unwanted act that will raise people's temperature".

"It is a huge mistake for him to choose to do that because it will further isolate his people ... who are desperate for food not missile launches, who are desperate for opportunity not for a leader who wants to flex his muscles in this manner," he said.

Mr Kerry said that in his talks with South Korean President Park Geun-hye she had made clear her "bright vision" of a peaceful Korean peninsula free of nuclear weapons.

"We are prepared to work with conviction that relations between the North and South can improve and can improve very quickly," he said.

"The world will be much better off if the leaders of the North, and one leader in particular, can make the right decision."On Saturday Mr Kerry will move on to China. He said he would urge leaders there to use their influence to rein in Pyongyang's aggression. He will then travel to Japan.

Mr Kerry said it was "clear to everybody in the world that no country in the world has as close a relationship or as significant an impact on [North Korea] than China", and that talks there would aim to "lay out a path that will defuse this tension".

His hope, he said, was that from talks in China and Japan "we will find the unity necessary to provide a very different set of alternatives for how we can proceed and ultimately diffuse this situation".

On Thursday, China carried out a civilian emergency drill in a town near its border with the North.

China's state media said the half-hour exercise covered evacuations and responses to an air raid and was aimed at raising public awareness of disaster prevention and relief.

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.