RoK President Moon Jae-in (L) and his wife Kim Jung-sook arrive at joint Base Andrews in Maryland on May 21, 2018, for a two-day visit to Washington that will include a bilateral summit with US President Donald Trump.
Chung Eui-yong, chief of the presidential National Security Office, made the remarks aboard Air Force One bound for Washington, where the Republic of Korea (RoK) President Moon Jae-in is to hold a summit with US President Donald Trump on May 22.
"We believe there is a 99.9 percent chance the North Korea (DPRK)-US summit (set for June 12 in Singapore) will be held as scheduled," Chung told reporters. "But we're just preparing for many different possibilities."
Moon, who held historic summit talks with Kim on April 27, is expected to share with Trump his experiences of dealing with the DPRK's leader so as to help Trump prepare for his own talks with Kim set for June 12 in Singapore.
Moon has already held two telephone conversations with Trump since his summit with Kim.
DPRK has recently ratcheted up its criticism of both Seoul and Washington over the allies' joint military exercise. The DPRK has threatened to reconsider holding its first-ever summit with the United States and also suspended its high-level dialogue with Seoul originally set for last week.
Meanwhile, a group of roughly two dozen journalists from Western and Chinese news organisations departed for DPRK on May 22 to witness the closure of its nuclear test site, an indication that the shut-down will go ahead.