Spurning a six-hour flight, North Korea leader Kim Jong-un is crawling across China on a two-day journey that will take him to Vietnam for a second summit with US President Donald Trump.

Mr 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.

Why Mr Kim has chosen to ride rails is a matter of speculation, but one theory posits that the armoured train’s communication system allows him to keep a closer watch on events in Pyongyang in his absence from the North Korean capital.

Late on Saturday, an Associated Press reporter saw a green and yellow train similar to one used in the past by Mr 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 in 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 nuclear disarmament and the removal of US-led sanctions against the North.

Vietnam’s foreign ministry announced on Saturday that Mr 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 Mr Trump, experts say Mr 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.

Kim Jong-un’s second rendezvous with Donald Trump will see some hard bargaining. Photo: Getty

While Mr 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.

-with AAP