Kim left Pyongyang for China on Monday and crossed into China early Tuesday morning, North Korea's state newspaper Rodong Sinmun said, en route to the largest multilateral diplomatic event he has ever been to.

Kim is expected to attend the military parade in Beijing on Wednesday, joining Chinese President Xi Jinping and other leaders, including Russian President Vladimir Putin and Iranian President Masoud Pezeshkian.

Rodong Sinmun published photographs of Kim and his entourage, including Foreign Minister Choe Son Hui, who has been involved in Pyongyang's diplomacy on weapons developments for more than two decades.

In one photograph, Kim can be seen standing next to the dark green train with a cigarette in his hand.

In another, he is seated at a desk inside the train with a North Korean flag behind him and with a closed laptop computer in a carriage ornately furnished with wood paneling.

The train appeared to be similar to the bulletproof train he has used before to travel to other countries.

Before crossing into China, Kim on Monday visited a missile laboratory that is researching carbon fiber composite materials to be used in engines for intercontinental ballistic missiles, state media KCNA said.

Kim's visit to a missile laboratory just ahead of the multilateral summit appeared highly symbolic, experts said.

The visit is geared toward "showing off [North Korea's] status as a nuclear power" just before "standing alongside Xi and Putin, which is intended to suggest support for North Korea as a nuclear state," said Hong Min, North Korea analyst at the Korea Institute for National Unification.

Hong said it also sent a message to the US about North Korea's "will to advance nuclear weapons and the status of a nuclear power that is difficult to reverse”.

North Korea on Monday expressed support for remarks made by Xi at a summit calling for fairer global governance, adding that cooperation between North Korea and China will grow to pursue such a value, according to a vice foreign minister in comments posted on the North Korean foreign ministry's website.

Xi on Monday pressed his vision for a new global security and economic order that prioritizes the "Global South", in a direct challenge to the United States, during a summit that included the leaders of Russia and India.

South Korean President Lee Jae Myung, who has been seeking to improve ties with Pyongyang, was receiving real-time reports about Kim's trip to China, the presidential office said. Seoul is sending National Assembly speaker Woo Won-shik to Beijing, but the country's Unification Ministry said it was unsure whether he would be able to hold bilateral talks with the North Korean leader on the sidelines of the event.