Speaking to reporters outside the White House, Trump said president Barack Obama told him before he took office that the "most dangerous problem" for the United States was North Korea's nuclear weapons program.

"I have solved that problem," Trump said. "Now we're getting it memorialized and all but that problem is largely solved."

"We signed a very good document," he said. "But more importantly than the document I have a good relationship with Kim Jong Un.

"That's a very important thing. I can now call him," he said. "I gave him a very direct number. He can now call me if he has any difficulty. We have communication.

"People are shocked," the president continued. "They thought Trump was going to get in, he's going to start throwing bombs all over the place.

"It's actually the opposite."