Kim will join Chinese President Xi Jinping and other world leaders including Russia's Vladimir Putin for a huge spectacle in which China will showcase its military prowess, with troops marching in formation, flypasts and other high-tech fighting gear.

More than 25 leaders will also attend Wednesday's parade centred on Beijing's Tiananmen Square to mark 80 years since the end of World War II.

Millions of Chinese people were killed during a prolonged war with imperial Japan in the 1930s and 40s, which became part of a global conflict following Tokyo's attack on Pearl Harbor in 1941.

China has touted the parade as a show of unity with other countries, and Kim's attendance will be the first time he has been seen with Xi and Putin at the same event.

The North Korean leader's special train passed into China early Tuesday, South Korea's Yonhap news agency reported, citing the North's state-run radio service, KCBC.

His appearance in China "formalises the China-Russia-North Korea trilateral (relationship) to the public", Soo Kim, a geopolitical risk consultant and former CIA analyst, told AFP.

Kim enjoyed a brief bout of high-profile international diplomacy from around 2018, meeting US President Donald Trump and then South Korean president Moon Jae-in multiple times.

But he withdrew from the global scene after the collapse of a summit with Trump in Hanoi, Vietnam, in 2019.

Kim stayed in North Korea throughout the Covid-19 pandemic, but met Putin in Russia's far east in 2023.

- Flags, flowers and fanfare -

Security around Beijing has tightened in recent days and weeks, with road closures, military personnel stationed on bridges and street corners, and miles upon miles of white barriers lining the capital's wide boulevards.

Art installations with flowers, doves and an emblem showing the Great Wall of China with "1945-2025" have cropped up around the city, and on Tuesday morning Chinese flags flew in residential neighbourhoods.

Officials have been tight-lipped over the list of hardware to be displayed at the parade, but military enthusiasts have already spotted significant new systems, including what is rumoured to be a gigantic laser weapon.

Wednesday's event caps a bumper week of diplomacy for President Xi, who on Sunday and Monday hosted a slew of Eurasian leaders for a summit in the northern port city of Tianjin aimed at putting China front and centre of regional relations.

The club of 10 countries -- named the Shanghai Cooperation Organisation (SCO) -- touts itself as a non-Western style of collaboration in the region and seeks to be an alternative to traditional alliances.

During the summit, Xi slammed "bullying behaviour" from certain countries -- a veiled reference to the United States -- while Putin defended Russia's Ukraine offensive, blaming the West for triggering the conflict.

Many of the guests from the Tianjin gathering, including Putin, Belarusian President Alexander Lukashenko and several other leaders will join Xi and Kim for the parade in Beijing.