Kerry presses China to help end N. Korea tensions

US Secretary of State John Kerry on Saturday pressed North Korea's patron China to help defuse sky-high nuclear tensions, telling President Xi Jinping the world was facing a "critical" moment.

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 there are fears it could soon fuel the crisis by firing a medium-range missile.

"Mr President, this is obviously a critical time with some very challenging issues," Kerry told China's new leader in the Great Hall of the People, on the second leg of an Asian tour that has been swamped by the Korea crisis.

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.

Kerry 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.

Washington is seeking to persuade Beijing to help rein in the bellicose threats from North Korean leader Kim Jong-Un, and bring Pyongyang back to the negotiating table over its suspect nuclear programme.

Without giving specifics, he said after his meeting with Xi that they "could not have had a more constructive and forward-leaning conversation".

Earlier in Seoul Kerry said: "I think it's clear to everybody in the world that no country in the world has as close a relationship or as significant an impact on the DPRK (North Korea) than China."

Beijing has backed Pyongyang since the 1950-53 Korean War and could wield tremendous leverage over the isolated communist regime thanks to the vital aid it provides, including almost all of its neighbour's energy imports.

But analysts say it is wary of pushing too hard for fear of destabilising North Korea, which could send waves of hungry refugees flooding into China and ultimately lead to a reunified Korea allied with the United States.

A Chinese city on the border with North Korea and Russia, Hunchun, held an air raid drill Friday but an official told AFP it was a routine exercise.

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 such as Syria and Iran, as well as trade.

The US Treasury Department on Friday said China's yuan currency remains "significantly undervalued" but stopped shy of accusing Beijing of manipulating the currency to gain an unfair trade advantage.

In opening remarks in the presence of reporters, Xi did not mention Korea 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".

"While accusing Pyongyang of reckless provocation and intolerable disregard of international wishes, Washington itself has also been fanning the flames," it said, citing a recent US military buildup in support of South Korea.

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

Despite intelligence reports that the North is ready to stage a highly provocative medium-range missile launch, Park has in recent days made some conciliatory overtures, saying the South should meet with the North.

While Kerry berated Pyongyang's "unacceptable" rhetoric and warned that any missile launch would be a "huge mistake", he also stressed US backing for Park's initiative.

In Beijing, the secretary of state also met with Premier Li Keqiang and Foreign Minister Wang Yi, and on Sunday will head to Tokyo.

Kerry said he hoped China, Japan and the United States can find the "unity" required to offer a "very different set of alternatives for how we can proceed and ultimately how we can defuse this situation" on the Korean peninsula.