Top US envoy begins Asia tour amid North Korea crisis

Tokyo: Washington's top diplomat was to fly into Tokyo on Wednesday to begin the first tour of Asian capitals under the shadow of North Korea's nuclear posturing.

Secretary of State Rex Tillerson has promised to take a tough line on Kim Jong-un's isolated regime as he talks to allies Japan and South Korea and to rival great power China.

The former oilman will also talk commerce after President Donald Trump's belligerent campaign rhetoric raised the spectre of a trade war between the United State and China.

But US officials confirmed North Korea's provocative behaviour would be "front and centre" as Tillerson meets Japan's Prime Minister Shinzo Abe and Foreign Minister Fumio Kishida on Thursday.

After Tokyo, it will be on to Seoul, a city already within range of North Korea's artillery and rocket batteries, for talks with the acting leader after a corruption scandal, saw President Park Geun-Hye ousted last week.

And then at the weekend, there will be his much anticipated trip to Beijing, amid reports that Tillerson will seek to finalise plans for Chinese leader Xi Jinping to visit Trump in April.

Here, he might get a positive response. In a news conference after the annual National People's Congress parliament meeting on Wednesday, Premier Li Keqiang said, "there are bright prospects for China-US cooperation."

He warned that US investments in China would bear the brunt of any trade war, and argued: "I believe whatever differences we have, we can still sit down and talk to each other and work together to find solutions."

But Tillerson will also have to broach the topic of North Korea's progress towards building an intercontinental ballistic missile (ICBM) that would be able to threaten US mainland cities or bases in the Pacific.

"He's going to have an opportunity at every stop to talk about next steps or what we do now, with respect to North Korea," acting State Department spokesman Mark Toner told reporters.

"I mean, it's obviously the looming challenge over our relations and, frankly, the security of the Korean Peninsula," he added in a news briefing before Tillerson left Washington.

China is perhaps the last country with significant leverage over North Korea, which has ignored several rounds of UN-backed sanctions targeting its banned nuclear weapons programme.

Beijing shares US concerns over Pyongyang's attempts to build an arsenal of nuclear devices but has been much more measured in its reaction to the ballistic missile program.

Xi's government has been reluctant to back any moves that might destabilise Kim's regime and is publicly more concerned about Washington's decision to deploy the THAAD missile defence program in South Korea.

Washington insists THAAD is a defensive system deployed to protect the South and US bases from North Korean missiles, but China fears its advanced radar system undermines China's nuclear deterrent.