"I know that a letter is being delivered to me, a personal letter from Kim Jong Un to me, that was handed at the border," Trump told reporters traveling with him to North Dakota.

"I think it's going to be a positive letter."

Trump also said Kim had made a "very positive" statement about him.

"That was a very positive statement, what he said about me," Trump said. "There's never been a more positive statement."

Kim on Thursday renewed his commitment to the goal of denuclearization in talks with a special envoy from Seoul, ahead of a summit planned in Pyongyang September 18-20 with South Korean President Moon Jae-in.

"The north and the south should further their efforts to realize the denuclearization of the Korean peninsula," KCNA quoted Kim as saying.

Kim also voiced his continuing trust in Trump, according to Seoul's envoy -- which prompted the US president to tweet out his thanks to the North Korean leader and vow to "get it done together."