US Secretary of State John Kerry

The top diplomats of China and the US have pledged to work together on "challenges" presented by North Korea, after a series of nuclear war threats from Pyongyang.

Meeting Chinese leaders in Beijing on Saturday, US Secretary of State John Kerry said the two countries were committed to finding a peaceful way to ensure a nuclear-free Korean peninsula.

Beijing is Pyongyang's main trading partner, financial backer and diplomatic ally, and is seen as having unique leverage over the government of Kim Jong-un.

"To properly address the Korean nuclear issue serves the common interests of all parties," China's State Councillor Yang Jiechi, who is in charge of Beijing's foreign policy, said.

"China will work with other relevant parties including the United States to play a constructive role."

Neither Yang nor Kerry gave details of any concrete measures that had been agreed. But Kerry said they would have "further discussions to bear down very quickly with great specificity on exactly how we will accomplish this goal".

The world is facing a "critical time", Kerry told China's President Xi Jinping, also citing Iran's nuclear programme and the conflict in Syria.

"Mr President, this is obviously a critical time with some very challenging issues," Kerry told Xi.

"Issues on the Korean peninsula, the challenge of Iran and nuclear weapons, Syria and the Middle East, and economies around the world that are in need of a boost."

'Historical stage'

Kerry arrived from South Korea to press Beijing to help defuse soaring nuclear tensions on the Korean peninsula ahead of an expected missile launch by the North, which conducted a nuclear test in February.

But Xi did not refer to the Korean peninsula or other issues raised by Kerry in his opening remarks at the meeting, instead saying that the US-China relationship was "at a new historical stage and has got off to a good start".

As North Korea's sole major ally, Kerry said ahead of his visit that China has a unique ability to influence the impoverished, isolated state.

"There is no group of leaders on the face of the planet who have more capacity to make a difference in this than the Chinese, and everybody knows it, including, I believe, them," Kerry said in Seoul on Friday.

"They want to see us try to reach an amicable resolution to this."

Analyst of North Korean affairs Hazel Smith told Al Jazeera that both the US and China want to keep the situation on the Korean peninsula under control but that they each think the other can do more.

“China sees that the United States has a responsibility as well as North Korea to get in some form of serious diplomatic relations about bringing to an end the overall conflict between the two protagonists, so that these military situations don’t reoccur,” Smith said.

Kerry's visit to Asia, which will include a stop in Tokyo on Sunday, takes place after weeks of North Korean threats of an impending war since the imposition of new UN sanctions in response to its third nuclear test.

Separately, NATO chief Anders Fogh Rasmussen visited South Korea for the first time. He toured the Demilitarised Zone (DMZ) that separates the North and the South on Saturday, and viewed North Korean territory from the observatory located within the DMZ.