Pyongyang, already under a raft of international sanctions for its nuclear weapons programme, has repeatedly denied supplying arms to Russia -- though that stance could change in the coming days, experts say.
Kim, wearing a black suit and flanked by uniformed defence officials, waved from the doorway of his heavily armoured private train with green-and-gold livery as it departed Pyongyang station Sunday evening, with the Kremlin confirming Putin would meet Kim later this week in Russia's far east.
The United States on Monday described Putin as "begging for assistance" by meeting with an "international pariah" in Kim, and renewed warnings that any arms deal could trigger US sanctions.
AFP takes a look at what we know about North Korean-Russian ties:
- What can North Korea offer Russia? -
Earlier this month, Washington said that despite its denials, Pyongyang supplied infantry rockets and missiles to Moscow in 2022 for use by the privately controlled Wagner military group.
Joseph Dempsey, a researcher at the International Institute for Strategic Studies, said Moscow would be most interested in artillery shells that can be easily integrated.
"North Korea likely represents the largest stockpiles of Soviet-era legacy artillery shells and artillery that could be used to restock depleted Russian inventories from the Ukraine conflict," Dempsey told AFP.
Once the summit is over -- regardless of its outcome -- "the new Cold War structure between South Korea, the United States, and Japan versus North Korea, China, and Russia will intensify", Yang Moo-jin, president of the University of North Korean Studies in Seoul, told AFP.
"If that happens, denuclearisation and peaceful prosperity on the Korean Peninsula will be a long way off," even more so than now, he added.
- What does North Korea want in return? -
Analysts say Russia has everything that impoverished North Korea needs.
"Russia is a food export country, a fertiliser export country, an energy export country," said Cho Han-bum, senior research fellow at the Korea Institute for National Unification.
Pyongyang could also seek the transfer of "key technologies, knowledge and manufacturing capacity for North Korea's arms industry to advance and be more sustainable", added Dempsey.
A 2022 UN report highlighted the role of a North Korean diplomat in Moscow in procuring a range of ballistic missile technologies, and going as far as attempting to get three tonnes of steel for Pyongyang's submarine programme.
North Korea could also get diplomatic gains from the deal by sending a message to China.
"Since the Cold War era, North Korea has always practised the so-called 'pendulum diplomacy' between China and the Soviet Union, going back and forth to maximise its benefit," said Park Won-gon, a professor at Ewha University.
"I can see some of that being practised now."
- What about summits in the past? -
Russia, a historical ally of Pyongyang, was a crucial backer of the isolated country for decades and their ties go back to the founding of North Korea.
But the Soviet Union reduced funding to the North as it began to seek reconciliation with Seoul in the 1980s. Pyongyang was hit hard by its demise in 1991.
The Russian Federation and the North held their first summit in 2000, when a joint declaration -- which focused on economic cooperation and diplomatic exchanges -- was signed.
The signing of the agreement between Russian President Vladimir Putin and the late Kim Jong Il, the father and predecessor of Kim Jong Un, marked a significant milestone in revitalising bilateral relations after a period of stagnation.
Kim Jong Un made his first official visit to Russia in 2019, as he was seeking closer ties with the North's traditional ally amid a nuclear deadlock with Washington.
Kim and Putin released no joint statement at the time.
But Kim has been steadfast in his support for Moscow's invasion of Ukraine, including, Washington says, supplying rockets and missiles.
Putin in July hailed Pyongyang's "firm support for special military operations against Ukraine".
- What would a Pyongyang-Moscow deal mean? -
The White House warned last week that North Korea would "pay a price" if it supplies Russia with weaponry for the conflict in Ukraine.
Defying the warnings, Kim left North Korea for Russia on Sunday, according to the official Korean Central News Agency (KCNA).
He was accompanied by top North Korean military officials, including those in charge of weapons production and space technology, it added.
Cheong Seong-chang, researcher at the Sejong Institute, told AFP that, were North Korea to expand military cooperation with Russia, "there is an increased likelihood of prolonged conflict in Ukraine".
And Pyongyang's reward for aiding Moscow could mean that "advancements in North Korea's nuclear submarine and reconnaissance satellite development might then progress at a faster pace", he said.
If Moscow and Pyongyang indeed proceed with shipments of arms, locating them will become the "responsibility of the international community" to hold them accountable.
"In case of shells, the North could transport them to Moscow via trains for their relatively small size," Cheong said.
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