Donald Trump has told US troops at South Korea's Osan Air Base that his "unexpected" meeting with North Korean leader Kim Jong Un was "great."

"That's a great country with tremendous potential," the US President said. "I have a good relationship with chairman Kim, they were giving us a great briefing at the DMZ."

As for his walk into North Korean territory, Trump called it a "historic moment and a very good moment."

"Everybody was so happy and many people I noticed from Korea were literally in tears, crying."