North Korea's leader Kim Jong Un waves from his vehicle after arriving at the Dong Dang railway station, Vietnam, at the border with China, February 26, 2019.

North Korea's leader Kim Jong Un arrives at the Dong Dang railway station, Vietnam, at the border with China, February 26, 2019.

North Korea's leader Kim Jong Un gets into his vehicle after arriving at the Dong Dang railway station, Vietnam, at the border with China, February 26, 2019.

North Korea's leader Kim Jong Un waves as he arrives at the Dong Dang railway station, Vietnam, at the border with China, February 26, 2019.

North Korea's leader Kim Jong Un waves as he arrives at the Dong Dang railway station, Vietnam, at the border with China, February 26, 2019. REUTERS/Athit Perawongmetha

Kim Yo Jong, sister of North Korea's leader Kim Jong Un and first vice department director of the ruling Workers’ Party’s Central Committee, arrives at the Dong Dang railway station, Vietnam, at the border with China, February 26, 2019.

U.S. President Donald Trump boards Air Force One for travel to a second summit meeting with North Korea's Kim Jong Un in Hanoi, Vietnam from Joint Base Andrews, Maryland, U.S., February 25, 2019.

North Korea's leader Kim Jong Un sits in his vehicle after arriving at the Dong Dang railway station, Vietnam, at the border with China, February 26, 2019.

North Korea's leader Kim Jong Un arrives at the Dong Dang railway station, Vietnam, at the border with China, February 26, 2019.

U.S. Secretary of State Mike Pompeo, left, speaks with U.S. Ambassador to Vietnam Daniel Kritenbrink, second from left, as he arrives at Noi Bai International Airport in Hanoi, Vietnam, Tuesday, Feb. 26, 2019, for a second summit between President Donald Trump and North Korean leader Kim Jong Un.