US Secretary of State John Kerry headed to Japan on Sunday after China and the United States vowed to work together to try to defuse nuclear tensions on the Korean peninsula.

After an intense day of diplomacy Saturday 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.

"The importance of the visit yesterday really cannot be overstated," Kerry told US embassy staff in Beijing on Sunday ahead of his departure for Tokyo.

"This is a critical time needless to say, being able to speak directly to my Chinese counterpart and try to focus on some very critical issues is of major importance."

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".

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".

Kerry told reporters 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".

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 added.

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 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 percent of its neighbour's energy imports, 80 percent of its consumer goods and 45 percent 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," Kerry told 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.

Strains in the relationship between the United States and China, the world's top two economies, have been simmering on an array of diplomatic fronts including Syria and Iran, as well as trade.

The US Treasury Department on Friday said China's yuan remains "significantly undervalued" but stopped short of accusing Beijing of manipulating the currency.

But Kerry in his talks also agreed "to try to accelerate" talks on economic issues, particularly the problems faced by Chinese and American businesses trying to work in the other nation.

Xi did not mention Korea at the start of his talks with Kerry, but said the China-US relationship was "at a new historical stage and has got off to a good start" since his ascension as head of state last month.

But in a commentary issued minutes later, China's official Xinhua news agency said America's strategic "pivot to Asia" could breed mistrust, and Washington should "help seek reasonable and workable solutions to regional issues".

Kerry again warned that any missile launch by the North in the coming days would be seen as "provocative".

But he raised the possibility that "if the threat disappears" and North Korea denuclearises, Washington could stand down its forces as it would no longer have "the same imperative... to have that kind of robust, forward-leaning posture".

On Sunday Kerry heads to Japan, which deployed Patriot missiles around Tokyo this week as anticipation of the North's missile launch mounted.