Trump, speaking to Reuters on Thursday, said he wanted to peacefully resolve the crisis, 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, according to China’s foreign ministry.

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 neighbor’s pursuit of nuclear weapons and long-range ballistic missiles in violation on U.N. resolutions.

Trump lavished praise on Chinese President Xi Jinping for trying to rein in Pyongyang.

“I believe he is trying very hard. He certainly doesn’t want to see turmoil and death. He is a good man,” Trump said.

U.S. Secretary of State Rex Tillerson said on Thursday China has asked North Korea not to conduct any more nuclear tests, and that Beijing had warned it would impose unilateral sanctions if the isolated state went ahead.

“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 U.N 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.

China has long promoted dialogue to resolve the Korean nuclear issue as North Korea has repeatedly threatened to destroy the United States.

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

Separately, Russian President Vladimir Putin and Japanese Prime Minister Shinzo Abe called on North Korea and other countries on Thursday to avoid behavior or rhetoric that could increase tensions around Pyongyang’s nuclear program.