There had been speculation that the hawkish adviser had been left out of Trump's delegation, because his name was not on a list of officials distributed by the White House.

Bolton's spokesman, Garrett Marquis, confirmed in an email that Bolton was already in the Vietnamese capital ahead of the president's scheduled 9:15 p.m. arrival.

Bolton is known for his hardline stance on North Korea. He has repeatedly expressed skepticism over whether the regime will abandon its nuclear weapons program. But in recent months he has softened his public remarks in apparent consideration of Trump's eagerness to engage with Kim.

The adviser had been scheduled to visit South Korea last week for talks with his South Korean and Japanese counterparts on the summit but later canceled the trip to focus on events in Venezuela. (Yonhap)