North Korean leader Kim Jong Un arrived in Russia on Tuesday ahead of a meeting with President Vladimir Putin that the United States has warned could see an arms deal to support Moscow's assault on Ukraine.

Making a rare foreign trip and his first since the pandemic, Kim was seen stepping onto a red-carpeted train platform before meeting Natural Resources Minister Alexander Kozlov.

Kim and Putin are expected to meet at an unspecified location in Russia's Far East later this week, the Kremlin has said.

Putin is currently attending the Eastern Economic Forum in Vladivostok, the Pacific port city closest to the North Korean border, though there has been no indication that the internationally isolated pair would hold their talks there.

Reporters granted access to the Russian leader at the forum refrained from asking Putin details of the visit but he told journalists he would soon travel to the Vostochny Cosmodrome, a Russian spaceport some 1,000 kilometres (620 miles) from Vladivostok.

"I've got my programme there, and when I get there you'll know," he was quoted by Russian news agencies as saying.

Kim received a VIP welcome from a military honor guard, with the national anthems of both countries playing, as he arrived in the Russian border town of Khasan Tuesday morning, the state-run KCNA news agency said.

Kim told his Russian hosts that his visit was a "clear manifestation" of North Korea "prioritizing the strategic importance" of its ties with Russia, KCNA said.

The agency did not specify when or where Kim would meet with Putin, saying only that after the arrival ceremony the North Korean leader "left for his destination."

Experts say Moscow will likely seek artillery shells and antitank missiles from North Korea, which wants advanced satellite and nuclear-powered submarine technology in return.

Siemon Wezeman, a senior researcher at the Stockholm International Peace Research Institute, said it was "entirely possible" North Korea had large stocks of ammunition that could be used by Russia.

"Whether any deal is struck remains to be seen," he said.

"We will not know for sure until there is hard evidence that Russia has used North Korean arms and ammunition on the battlefield in Ukraine," he added.

The White House warned last week that North Korea would "pay a price" if it supplied Russia with weaponry for the conflict in Ukraine.

Kremlin spokesman Dmitry Peskov said Putin and Kim would "cooperate on sensitive areas that should not be the subject of public disclosure and announcements".

Steadfast allies 

Kim is travelling to Russia with his top military officials including Korean People's Army Marshal Pak Jong Chon and Munitions Industry Department Director Jo Chun Ryong, analysts said.

This indicates a Putin-Kim summit "is likely to heavily focus on Russia and North Korea's possible military cooperation," Yang Moo-jin, president of the University of North Korean Studies in Seoul, told AFP.

Moscow sent Defence Minister Sergei Shoigu to Pyongyang in July. He has recently mooted bilateral joint naval drills.

Kim has been steadfast in his support for Moscow's assault on Ukraine, including, Washington says, supplying rockets and missiles.

But both Moscow and Pyongyang have denied North Korea has or will supply arms to Russia, which has eaten into its vast stockpiles of munitions since it launched its Ukraine offensive early last year.

Kim has not travelled outside North Korea since the start of the coronavirus pandemic. His last proper overseas trip was in 2019, also to Russia to meet Putin.

'Begging' for help 

"North Korea has the crude ammunition that Putin needs for his illegal war in Ukraine, while Moscow has submarine, ballistic, and satellite technologies that could help Pyongyang leapfrog engineering challenges it suffers under economic sanctions," said Leif-Eric Easley, a professor at Ewha University in Seoul.

On Monday, the United States described Putin as desperate in seeking a meeting with Kim.

"Having to travel across the length of his own country to meet with an international pariah to ask for assistance in a war that he expected to win in the opening month, I would characterize it as him begging for assistance," State Department spokesman Matthew Miller said.

"I will remind both countries that any transfer of arms from North Korea to Russia would be in violation of multiple United Nations Security Council resolutions," he added.

Washington has said Russia could use weapons from North Korea to attack Ukrainian food supplies and heating infrastructure heading into winter to "try to conquer territory that belongs to another sovereign nation".

Andrei Lankov, a North Korea expert at Kookmin University in Seoul, told AFP that the upcoming meeting was part of Moscow's "gentle diplomatic blackmail" of Seoul because Russia did not want South Korea to supply weapons to Kyiv.

Seoul is a major arms exporter and has sold tanks to Kyiv's ally Poland, but longstanding domestic policy bars it from selling weapons into active conflicts.

"The major worry of the Russian government now is a possible shipment of the South Korean ammunition to Ukraine, not just one shipment but a lot of shipments," Lankov said.