Kim was accompanied by Kim Yong Chol, who has been a key negotiator in talks with the US, and Kim Yo Jong, the leader's sister, the North's official Korean Central News Agency reported on Sunday.

Late on Saturday, an Associated Press reporter saw a green and yellow train similar to one used in the past by Kim cross into the Chinese border city of Dandong via a bridge.

The Trump-Kim meeting is slated for Wednesday and Thursday in Hanoi.

Their first summit last June in Singapore ended without substantive agreements on the North's nuclear disarmament and triggered a months-long stalemate in negotiations as Washington and Pyongyang struggled with the sequencing of North Korea's nuclear disarmament and the removal of US-led sanctions against the North.

Kim's overseas travel plans are routinely kept secret.

It could take more than two days for the train to travel thousands of kilometres through China to Vietnam.

Vietnam's Foreign Ministry announced on Saturday that Kim would pay an official goodwill visit to the country "in the coming days" in response to an invitation by President Nguyen Phu Trong, who is also the general secretary of Vietnam's ruling Communist Party.

In his upcoming meeting with Trump, experts say Kim will seek a US commitment for improved bilateral relations and partial sanctions relief while trying to minimise any concessions on his nuclear facilities and weapons.

While Kim wants to leverage his nuclear and missile program for economic and security benefits, there continue to be doubts on whether he's ready to fully deal away an arsenal that he may see as his strongest guarantee of survival.

Last year, North Korea suspended its nuclear and long-range missile tests and unilaterally dismantled its nuclear testing ground and parts of a rocket launch facility without the presence of outside experts, but none of those steps were seen as meaningful cutbacks to the North's weapons capability.

While North Korea has repeatedly demanded that the US take corresponding measures, including sanctions relief, Washington has called for more concrete steps from Pyongyang toward denuclearisation.

Australian Associated Press