US Secretary of State John Kerry arrived in Japan on Sunday to discuss nuclear tensions on the Korean peninsula after securing vital support from China to help defuse the weeks-long crisis.

US Secretary of State John Kerry arrived in Japan on Sunday to discuss nuclear tensions on the Korean peninsula after securing vital support from China to help defuse the weeks-long crisis.

He was due to meet Japanese Foreign Minister Fumio Kishida later in Tokyo, which has deployed Patriot missiles around the capital in anticipation of a missile launch by the North.

Kerry's visit follows an intense day of diplomacy Saturday in Beijing, where he warned Chinese leaders including President Xi Jinping that the stakes were high as China's erratic ally North Korea threatens a missile launch.

"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.

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

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 secretary of state previously held 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.