North Korea is running the risk of a "major, major conflict" with the United States, Donald Trump has warned.

In the latest sign of significantly heightened tension between the two nations, the president described concerns over the isolated state's nuclear ambitions as his biggest global worry.

Fears are growing in the White House that Pyongyang is close to developing nuclear warheads capable of hitting the US mainland.

Tensions in past weeks have increased with both nations carrying out military drills in shows of strength on the Korean peninsula.

President Trump, who is pressing for tighter sanctions on the one-party state, said he would like to resolve the situation difficulty but that "it's very difficult".

North Korea is trying to develop nuclear weapons. Credit: AP

"There is a chance that we could end up having a major, major conflict with North Korea. Absolutely," President Trump said in an Oval Office interview ahead of his 100th day in power.

"We'd love to solve things diplomatically but it's very difficult."

President Trump praised the contribution of China, saying President Xi Jinping was trying "very hard" to rein in Kim Jong-un.

Earlier on Friday, US Secretary of State Rex Tillerson revealed that China had warned Pyongyang it would carry out unilateral sanctions against its neighbour if the isolated state carried out another nuclear test.

The move, not confirmed by Beijing, would represent one of China's toughest threats yet against its defiant ally.

In recent weeks, President Trump has urged China to be more firm with North Korea over dismantling its weapons programme.

Rex Tillerson said that China had threatened sanctions. Credit: AP

President Trump insisted he wanted to peacefully resolve the crisis, a path that he and his administration are emphasising by preparing a variety of new sanctions while not taking the military option off the table.

"I believe he [Jinping] is trying very hard. He certainly doesn't want to see turmoil and death," the president said.

"He doesn't want to see it. He is a good man. He is a very good man and I got to know him very well.

"With that being said, he loves China and he loves the people of China. I know he would like to be able to do something, perhaps it's possible that he can."

Donald Trump and Chinese premier Xi Jinping met last month. Credit: AP

In an interview with Fox News, Mr Tillerson revealed that China was pressing the Korean regime to refrain from carrying out further nuclear tests.

He noted that major commemorations in the country had passed in recent days without a feared nuclear test or test of any intercontinental ballisticmissile.

"And in fact 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," Mr Tillerson said.

He did not, however, specify what those sanctions might look like - and they have not been confirmed by China.

On Friday, Mr Tillerson is due to chair a meeting with UN Security Councilwhere he said he would stress the need for members to fully implement existing sanctions and discuss ways to further pressure Pyongyang.

Firing drills have been carried out on both sides of the Korean peninsula. Credit: AP

Asked if he considered Kim to be rational, President Trump noted he had taken over the dictatorial regime from his father - Kim Jong-il - at an early age.

"He's 27 years old. His father dies, took over a regime. So say what you want but that is not easy, especially at that age," he said.

"I'm not giving him credit or not giving him credit, I'm just saying that's a very hard thing to do.

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

Mr Tillerson himself described Kim as not "insane".

He told Fox News that intelligence sources suggested leader Kim was "not crazy," indicating he might be a rational actor who could be negotiatedwith.

"He may be ruthless, he may be a murderer. He may be someone who in many respects we would say by our standards is irrational," Mr Tillerson said.

"But he is not insane."