WASHINGTON (AFP) - The often difficult relations between the world's greatest powers, China and the United States, appear to be improving again as the pair work to resolve the North Korea nuclear standoff.

Washington, and in particular US President Donald Trump, have until very recently been critical of Beijing's cautious approach to its belligerent and nuclear-armed neighbour.

But, with a new round of UN-backed sanctions in place, the mood has improved, and US Secretary of State Rex Tillerson set off on Thursday (Sept 28) for Beijing for talks with senior Chinese leaders.

Tillerson is scheduled to confer this weekend with President Xi Jinping, State Councillor Yang Jiechi, who is the country's ranking diplomat, and Foreign Minister Wang Yi. The goal of these meetings is to prepare Trump's first visit to China as president, scheduled for November as part of a tour of Asia.

"But certainly North Korea will be on the table for discussion," Tillerson told reporters on Wednesday.

Washington hopes that a combination of UN and US sanctions and a strong signal from Pyongyang's main trade partner China will convince Kim Jong Un's regime of its isolation.

If the young autocrat then tones down his war of words with a no less provocative Trump and halts his nuclear and missile tests, he might be drawn to the table to discuss disarmament.

It will be a tough diplomatic slog. But while some US officials tout a "military option" and Trump has threatened to "totally destroy" North Korea, it seems to be the only game in town.

China's cooperation, however, will be key. And Trump, a proponent of a muscular "America first" foreign policy, has sometimes chafed at having to consider Beijing's diplomatic sensitivities.

Nevertheless, high-level ties have remained strong. Trump hosted Xi at his Florida golf resort early in his presidency, and Tillerson is making his second Beijing visit.

EFFORTS ON BOTH SIDES

As Tillerson packed his bags, his acting assistant secretary for East Asia Susan Thornton told skeptical US lawmakers that China appears to be on board with the plan to squeeze Pyongyang.

"We are working closely with China to execute this strategy and are clear-eyed in viewing the progress - growing, if uneven - that China has made on this front," she said.

"We have recently seen Chinese authorities take additional actions," she said, referring to new controls on the cross-border trade and finance that is North Korea's economic lifeline.

As late as last month, Trump was still excoriating China in his intemperate tweets for "not doing enough" to isolate Pyongyang, but this week he was much more measured.

"I applaud China's latest action to restrict its trade with North Korea," he said on Tuesday, singling out Xi personally for thanks.

"In particular, I applaud China for breaking off all banking relationships with North Korea, something people would have thought unthinkable even two months ago."

On Thursday, China added another layer to the apparently increasingly severe sanctions it is imposing on its neighbour, ordering North Korean firms on its territory to close by January.

The announcement came days after China confirmed it will limit exports of refined petroleum products to North Korea from October 1 while banning imports of textiles from its neighbour.

WHY THE CHANGE OF HEART?

Not everyone in Washington is convinced of China's sincerity.

Some hawks continue to push for more and more serious penalties for Chinese banks accused of facilitating transactions for North Korea, to make it clear the sanctions must hold.

And others suggest that Tillerson's push for talks with Kim, even talks backed up with the threat of coercive sanctions and military options, is naive about the threat.

But some experts also see that China has come to its own realisation that North Korea's brazen antics are a danger to the stability and security balance of its own east Asian region.

The former Asia advisor to president Barack Obama's National Security Council, Jeffrey Bader, said Beijing would not want to see its rivals re-arm in the face of North Korean bravado.

"China understands that South Korea and Japan will not stand still in the face of North Korean provocations," said Bader, now a senior fellow at the Brookings Institution.

But he added: "As important in Chinese eyes as the change in the security posture of its neighbours has been what it sees as the sheer effrontery of Pyongyang."

Kim offended his giant neighbour, Bader concludes, by testing an apparent hydrogen bomb on the same day Xi hosted a summit of the Brics group of emerging powers.

"The Trump administration deserves some credit for helping to orchestrate this shift in China, though the causes are largely regional and indigenous," he said.

Whatever the underlying reason for Beijing's change of heart, Tillerson will try to build on it, preparing the way for Trump's visit.