Washington has opened channels to North Korea to find out if the regime is ready to talk about giving up its nuclear weapons, Secretary of State Rex Tillerson said today.

Speaking after a day of talks with China's President Xi Jinping and top diplomats, Tillerson told reporters that US officials are in touch with Pyongyang.

The disclosure follows an escalating war of words between US President Donald Trump and North Korean strongman Kim Jong-Un, and Tillerson issued a call for calm.

Asked how he could know whether the North would even contemplate responding to new sanctions by coming to the table, the US envoy said: "We are probing, so stay tuned."

Washington has no diplomatic ties with Kim's autocratic regime, and has been leaning on Beijing to rein in its neighbour's behaviour through tougher sanctions.

But Tillerson said US diplomats do not rely on China as a go-between in overtures to North Korea, and have themselves talked directly through "our own channels."

"We ask," he said. "We have lines of communication with Pyongyang. We're not in a dark situation, a blackout, we have a couple, three channels open to Pyongyang."

"We can talk to them, we do talk to them," he said.

The US has not ruled out the use of force to compel Pyongyang to halt missile and nuclear tests, and last week Trump threatened to "totally destroy" the country.

But privately senior figures admit the military options do not look promising, with ally South Korea's densely populated capital Seoul in range of the North's artillery.

- Rocket man -

Tillerson, meanwhile, has been a proponent of a campaign of "peaceful pressure", using US and UN sanctions and working with China to turn the screw on the regime.

But his efforts have been overshadowed by an extraordinary war of words, with Trump mocking Kim as "little Rocket Man" and Kim branding the US leader a "dotard".

Even as Tillerson met Xi and China's top diplomats State Councillor Yang Jiechi and Foreign Minister Wang Yi, the North's propaganda agency fired a new barrage of insults.

The statement proclaimed Trump an "old psychopath" bent on the "suicidal act of inviting a nuclear disaster that will reduce America to a sea of flames".

North Korea's rhetoric has been backed by a provocative series of ballistic missile tests and on September 3 it conducted its sixth and most powerful nuclear test.

In the decade since the North's first nuclear explosion it has made rapid progress in developing the kind of missile technology that would allow it to hit US targets.

Washington, backed by most of the international community, has declared North Korea's programme unacceptable, fearing that its own vast arsenal will not deter Kim from attack.

With the world on edge, fears are growing that a miscalculation from either side could trigger a renewed deadly conflict on the divided Korean peninsula.

Some recent tests saw North Korean missiles flying over Japan en route to the Pacific, and its latest underground detonation seems to have been of a powerful hydrogen bomb.

Observers have expressed concern that if the North carries out an atmospheric nuclear test over the ocean, Washington will feel obliged to take risky military action.

- Heated rhetoric -

But Tillerson said that decision would be up to Trump alone and that "as far as I know the commander in chief has issued no red lines."

Tillerson instead called for calm, singling out Pyongyang's missile tests for criticism, but not rushing to defend Trump's own heated rhetoric and bellicose tweets.

"The whole situation is a bit overheated right now. I think everyone would like for it to calm down," he said in response to a question about Trump's threats.

"I think if North Korea would stop firing all the missiles, that would calm down things a lot," he said.

Tillerson, who was in Beijing to plan for a summit that Xi will host for Trump in November, welcomed recent measures taken by China to crack down on its neighbour.

The opening remarks at his three meetings with Xi, Yang and Wang contained no reference to North Korea, but he said afterwards the issue had been discussed.

US leaders criticised Beijing for taking too soft a line with Pyongyang, but in recent weeks China has taken more drastic measures to cut off trade and finance.

And in public, at least, the Chinese officials were all smiles for their guest.

Xi touted what he said was his "good working relationship and personal friendship" with Trump and predicted that the summit "will be a special, wonderful and successful one".