WASHINGTON (NYTimes, Reuters)- President Donald Trump's campaign to rein in North Korea is about to get a lot more complicated, as a progressive new leader in South Korea and fraying ties with China leave the United States with fewer partners to press Kim Jong Un on his nuclear or missile programmes.

Mr Trump will get a first taste of this less friendly landscape on Thursday (June 29), when he welcomes South Korea's newly elected president, Mr Moon Jae In, to the White House for dinner and an Oval Office meeting the next day.

Mr Moon campaigned on an independent-minded foreign policy and is more interested in engaging with the North than Mr Trump's hawkish advisers. He also suspended further deployment of a US-made anti-missile system in South Korea.

White House officials hope to defuse any tensions by trying to build good personal rapport between Mr Trump and Mr Moon, a 64-year-old human rights lawyer who served a previous South Korean leader, Roh Moo Hyun, known for clashing with the United States over North Korea.

As he tries to win over Mr Moon, Trump already is pivoting to a harsher approach to China, according to administration officials. The United States has demanded that Beijing crack down on Chinese banks and companies that do business with North Korea. And it plans to move ahead soon with tariffs on steel imports that are aimed partly at China. In its latest human trafficking report, issued on Tuesday, the State Department also downgraded China to the worst category, along with Syria, Iran, Russia and North Korea. It said China was guilty of using forced labour from North Korea.

The tough new tone comes after Trump expressed disappointment with President Xi Jinping for his failure to do more to pressure North Korea to stop its provocative behaviour. Mr Trump earlier had soft-pedalled his grievances in return for Mr Xi's help on North Korea. But with China not stepping up, senior officials said, he will feel less constrained about confronting China on trade and other areas of dispute.

On Monday, before a meeting with Prime Minister Narendra Modi of India, Mr Trump reiterated his sense of crisis about Pyongyang.

"The North Korean regime is causing tremendous problems," he said, "and is something that has to be dealt with, and probably dealt with rapidly."

With North Korea, as with other foreign policy challenges, Mr Trump has tended to personalise the issue, emphasising his rapport with leaders and his ability to strike deals. The question is whether he will have better luck with Mr Moon than he has had with President Xi.

"This is going to be a more difficult relationship than we've had for a few years," said Mr Jeffrey A. Bader, a top Asia adviser to President Barack Obama.

Mr Moon, he predicted, would seek to restore trade with the North, and visit Pyongyang during his five-year term. But, Mr Bader added, "at the outset, he wants a good relationship with Trump."

The White House clearly wants the same.

The two-day visit, officials said, would include ceremonies, like a wreath-laying and a visit to the Korean War Veterans Memorial, that are intended to celebrate the alliance between the United States and South Korea.

Officials said they were encouraged by an interview with President Moon in The Washington Post last week, in which he said he would not necessarily cancel the deployment of the anti-missile system, known as the Terminal High-Altitude Air Defense, or THAAD.

South Korea, he said, was conducting an environmental impact assessment on it.

The Trump administration, one senior official said, wants to avoid being interposed between the South and the North, which happened during previous periods when Washington and Seoul clashed over how to deal with the North Korean government. For some in the White House, Mr Moon's election was a political sea change in South Korea - a victory, particularly for young voters, who want a different kind of relationship with their northern neighbour. Some have even taken to calling it South Korea's "Brexit".

Popular sentiment in South Korea towards China has deteriorated in recent years, and the Moon government is also likely to have a chilly relationship with Japan. That could complicate efforts by the Trump administration to coordinate regional pressure against the North. For the next few weeks, the focus in Washington may switch from pressuring North Korea to pressuring China.

Some administration officials are pushing for the White House to announce punitive measures on steel imports even before a meeting of the Group of 20 nations in Hamburg, Germany, early next month. But the timing will depend on how quickly the administration is able to finalise a Commerce Department study of the global steel market.

China accounts for only two per cent of steel exports to the United States, but its excess capacity drives down steel prices worldwide. Surplus Chinese steel, shipped to other countries, ends up in the United States in other manufactured products. The Trump administration has also shifted from cajoling to threats in its exchanges with the Chinese on North Korea.

At a recent high-level meeting in Washington, a person who was briefed on the session said, the Americans handed the Chinese a list of companies that were dealing with the North Koreans and told them "to handle it - or else".

"Trump is being returned to the reality of China that everyone who has ever worked on China policy knows is the case," said Mr Michael J. Green, who served as a top Asia adviser to President George W. Bush.

As pressure mounts on Pyongyang to rein in its nuclear and missile programmes, China National Petroleum Corp has suspended sales of fuel to North Korea over concerns the state-owned oil company would not get paid, three sources told Reuters. It is unclear how long the suspension will last.

A prolonged cut would threaten critical supplies of fuel and force North Korea to find alternatives to its main supplier of diesel and gasoline, as scrutiny of China's close commercial ties with its increasingly isolated neighbour intensifies.

CNPC and the Ministry of Commerce did not respond to requests for comment. China's Foreign Ministry declined immediate comment. North Korea's embassy in Beijing declined to comment.

North Korea's unprecedented pace of nuclear and ballistic missile tests has prompted China, which handles 90 per cent of North Korea's trade, to start squeezing Pyongyang. In February, Beijing suspended coal purchases until the end of the year, cutting off North Korea's main export revenue source. In 2016, North Korea sold 22.5 million tonnes of coal to China, worth about US$1.9 billion (S$2.6 billion), according to Chinese customs.