President Trump warned a "major, major conflict" with North Korea could result from the ongoing standoff over its nuclear and missile programs, but he said he hoped diplomacy would prevail.

In an exclusive Oval Office interview with Reuters ahead of the 100-day benchmark in his administration, Trump candidly acknowledged that the international situation could go off the rails.

"There is a chance that we could end up having a major, major conflict with North Korea. Absolutely," Trump said.

"There is a chance that we could end up having a major, major conflict with North Korea."
— President Trump

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

Trump also said South Korea should pay for the $1 billion U.S. Terminal High Altitude Area Defense (THAAD) anti-missile defense system in place to protect it from an attack from the north.

The wide-ranging interview also included Trump's plan to defeat the Islamic State terror network.

"I have to say, there is an end. And it has to be humiliation," he said, when asked about what the endgame was for defeating Islamist violent extremism.

But talk was dominated by the North Korea situation, which has vexed past presidents, but seems to have reached a fever pitch amid repeated threats and new tests of missiles by Kim Jong-un.

"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," Trump 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," he said.

Trump said Chinese President Xi Jinping is helping to mitigate the issue.

"I believe he is trying very hard. He certainly doesn’t want to see turmoil and death. 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."

Trump spoke just a day after he and his top national security advisers briefed U.S. lawmakers on the North Korean threat and one day before Secretary of State Rex Tillerson will press the United Nations Security Council on sanctions to further isolate Pyongyang over its nuclear and missile programs.

The Trump administration on Wednesday declared North Korea "an urgent national security threat and top foreign policy priority." It said it was focusing on economic and diplomatic pressure, including Chinese cooperation in containing its defiant neighbor and ally, and would remain open to negotiations.

U.S. officials said military strikes remained an option, but played down the prospect, though the administration has sent an aircraft carrier and a nuclear-powered submarine to the region in a show of force.

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

Reuters contributed to this report.