As North Korea ramps up talk of a nuclear attack, here's a quick look at how the US, Japan and South Korea can counter a missile launch.

CHINA and the United States have vowed to work together to try to defuse nuclear tensions on the Korean peninsula, as US Secretary of State John Kerry says the world stands at a "critical" juncture.

During an intense day of diplomacy in Beijing, Kerry warned Chinese leaders including President Xi Jinping that the stakes were high as China's erratic ally North Korea threatens a missile launch that would extend a weeks-long crisis.



China is Pyongyang's sole major ally and backer, and is widely seen as the only country with leverage to influence its actions - although it is also reluctant to risk destabilising the regime.

State Councillor Yang Jiechi, who is in charge of Beijing's foreign policy, said China was committed to "advancing the denuclearisation process on the Korean peninsula" and "will work with other relevant parties including the United States to play a constructive role".

Mr Kerry said China and the United States "must together take steps in order to achieve the goal of a denuclearised Korean peninsula" and were "committed to taking actions".

But neither side gave details of any specific measures, and the top US diplomat said there would be "very focused continued high-level discussions about the ways to fill in any blanks".



Mr Kerry said he wanted to ensure that Saturday's pledges were "not just rhetoric, but that it is real policy".



He predicted he would be making "many trips" to Beijing, hailing what he called "an extremely positive and constructive day... beyond what I anticipated in many regards".

NORTH KOREA'S FURY AT US WAR GAMES

A LOOK AT NORTH KOREA'S MISSILE POWER

The US Chairman of the Joint Chiefs of Staff Martin Dempsey is to visit Beijing this month along "with other members of the intel community", he said.



The secretary of state flew in from talks in South Korea with President Park Geun-Hye, where he offered public support for her plans to initiate some trust-building with the North.



The region has been engulfed by threats of nuclear war by Pyongyang in response to UN sanctions imposed over its recent rocket and nuclear tests, and Mr Kerry stressed that China, which has backed Pyongyang since the 1950-53 Korean War, holds a unique sway over it and leader Kim Jong-Un.



China is estimated to provide as much as 90 per cent of its neighbour's energy imports, 80 per cent of its consumer goods and 45 per cent of its food, according to the US-based Council on Foreign Relations.

But analysts say it is wary of pushing too hard for fear of a regime collapse sending waves of hungry refugees flooding into China and ultimately leading to a reunified Korea allied with the United States.



"Mr President, this is obviously a critical time with some very challenging issues," Mr Kerry told Mr Xi earlier in the Great Hall of the People, on the second leg of his Asian tour.

As well as "issues on the Korean peninsula", he cited Iran's nuclear ambitions, Syria and the Middle East, and the world's economic woes.

WHY ALL THE HUBBUB

Since the 1950-53 Korean War, North Korea has feared that Washington is intent on destroying the regime.

The US worries that Pyongyang will re-ignite the conflict with South Korea, and is uneasy because little is known about Kim Jong Un, the North's new, young leader, and considers him unpredictable.

Both sides have ratcheted up the rhetoric and military muscle moves in recent weeks. North Korea threatened a pre-emptive strike against the US, and conducted an underground nuclear test in February and a rocket launch in December.

The threats are seen as an effort to pressure Washington and Seoul to change their North Korean policies and convince the North's people that their new leader is strong enough to stand up to its foes.

US and South Korean troops have been conducting annual joint military drills in the region since early March, including bringing out nuclear-capable stealth bombers and fighter jets in what the US Air Force acknowledged was a deliberate show of force.

NORTH KOREAN MISSILES

North Korea has been steadily working to display an increasing capability to launch missiles. Last year it failed in an attempt to send a satellite into space aboard a long-range rocket.

A subsequent launch in December was successful, and that was followed by the country's third underground nuclear test on February 12.

US officials believe the North is preparing to test fire a medium-range "Musudan" missile.

And a section in a new Defense Intelligence Agency assessment concludes with "moderate confidence" that the North could deliver nuclear weapon by ballistic missiles. The report notes that the delivery system is still not considered reliable.

US RESPONSE

The North American Aerospace Defense Command, or NORAD, which has responsibility for US homeland defense, is watching the region via satellite and the USS Fitzgerald and the USS John S. McCain, Navy destroyers armed with sophisticated missile defense systems, have been positioned to best be able to detect and track a missile launch.

The US is confident it would be able to shoot a missile down, but would do so only if it appears to be a threat to America or its allies.

The US is also prepared to provide military assistance to South Korea in the event of any other type of attack by the North.

WHAT THEY'RE SAYING

Secretary of State John Kerry warned North Korea not to conduct a missile test, saying it will be an act of provocation that "will raise people's temperatures" and further isolate the country and its people.

President Barack Obama said his administration would "take all necessary steps" to protect American citizens and he urged Pyongyang to end its threats. North Korea has issued no specific warnings to ships and aircraft that a missile test is imminent. And the country has begun festivities celebrating the April 15 birthday of the country's late founder, Kim Il Sung, which is considered the most important national holiday.

China has been a longtime political, military and economic backer of North Korea and is considered to have more real leverage over the North.

US officials say there are indications Chinese leaders have become frustrated with Pyongyang's recent behavior and rhetoric.

In a positive development for the US, China agreed publicly to work with the US to achieve the goal of a nuclear-free Korean peninsula.