US President Donald Trump said a “major, major conflict” with North Korea was possible over its nuclear and ballistic missile programmes, while China said the situation on the Korean peninsula could escalate or slip out of control.

Trump, speaking to Reuters on Thursday, said he wanted to resolve the crisis peacefully, possibly through the use of new economic sanctions, although a military option was not off the table.

“There is a chance that we could end up having a major, major conflict with North Korea,” Trump said in an interview at the Oval Office.

“We'd love to solve things diplomatically but it's very difficult,” he said, describing North Korea as his biggest global challenge.

Chinese Foreign Minister Wang Yi said there was a danger that the situation on the Korean peninsula could escalate or slip out of control, China's foreign ministry said.

Wang made the comments in a meeting with a Russian diplomat on Thursday at the United Nations, the ministry said in a statement.

China, the only major ally of North Korea, has been increasingly uncomfortable in recent months about its neighbour's pursuit of nuclear weapons and long-range ballistic missiles in violation on UN resolutions. The United States has called on China to do more to rein in Pyongyang and Trump lavished praise on Chinese President Xi Jinping for his efforts, calling him “a good man”.

“I believe he is trying very hard. I know he would like to be able to do something. Perhaps it's possible that he can't. But I think he'd like to be able to do something,” Trump said.

US Secretary of State Rex Tillerson said on Thursday that China had asked North Korea not to conduct any more nuclear tests. Beijing had warned Pyongyang it would impose unilateral sanctions if it went ahead, he added.

“We were told by the Chinese that they informed the regime that if they did conduct further nuclear tests, China would be taking sanctions actions on their own,” Tillerson said on Fox News, without specifying what sanctions he was referring to.

Tillerson did not say when China made the threat and there was no immediate confirmation from Beijing. He is due to chair a meeting with UN Security Council foreign ministers on Friday, where he said he would stress the need for members to fully implement existing sanctions as well as possible next steps.

China banned imports of North Korean coal in February, cutting off its most important export, and Chinese media this month raised the possibility of restricting oil shipments to the North if it unleashed more provocations.

Missile defence, carrier group

In a show of force, the United States is sending the USS Carl Vinson aircraft carrier group to waters off the Korean peninsula, where it will join the USS Michigan, a nuclear submarine that docked in South Korea on Tuesday. South Korea's navy has said it will hold drills with the US strike group.

Admiral Harry Harris, the top US commander in the Pacific, said on Wednesday the carrier was in the Philippine Sea, within two hours' striking distance of North Korea if need be.

Harris also said a US missile defence system being deployed in South Korea to ward off any possible North Korean attack would be operational in coming days.

However, Beijing has been angered by the deployment of the Terminal High Altitude Area Defence, or THAAD, missile defence system, complaining that its radar can see deep into China and undermines its security.

Trump said in the interview he wants South Korea to pay the cost of the THAAD, which he estimated at $1 billion. South Korea, one of Washington's most crucial allies in the region, said the US would have to bear the cost, pointing to possible friction ahead.

The spokesman for Moon Jae-in, the frontrunner in South Korea's May 9 presidential election, said on Friday that deployment of THAAD should be “immediately suspended” ahead of a decision by the next government.

Trump has vowed to prevent North Korea from being able to hit the US with a nuclear missile, a capability experts say Pyongyang could have some time after 2020.

North Korea has conducted five nuclear tests and numerous missile tests, including one this month, a day before a summit meeting between Trump and Xi in Florida.

Any direct US military action would run the risk of massive North Korean retaliation and huge casualties in Japan and South Korea and among US forces in both countries.

Trump, asked if he considered North Korean leader Kim Jong Un to be rational, said he was operating from the assumption that he is rational. He noted that Kim had taken over his country at an early age.

“As to whether or not he's rational, I have no opinion on it. I hope he's rational,” he said.

In Moscow, Russian President Vladimir Putin and visiting Japanese Prime Minister Shinzo Abe called on North Korea and other countries on Thursday to avoid behaviour or rhetoric that could increase tensions around Pyongyang's nuclear programme.

North Korea, technically still at war with the South after their 1950-53 conflict ended in a truce, not a treaty, regularly threatens to destroy the US and says it will pursue its nuclear and missile programmes to counter perceived US aggression.