North Korea said Monday that leader Kim Jong Un would visit Russia and meet President Vladimir Putin, with the reclusive ruler's armoured train reportedly on its way to the border.

Experts suggest Putin is seeking artillery shells and antitank missiles from North Korea for Moscow's war in Ukraine, while Kim is reportedly in search of advanced technology for satellites and nuclear-powered submarines, as well as food aid for his impoverished nation.

Kim "will soon visit the Russian Federation at the invitation of... Putin," the North's official Korean Central News Agency said.

"The respected Comrade Kim Jong Un will meet and have a talk with Comrade Putin during the visit," it added.

The Kremlin also confirmed Monday that Kim would visit Russia "in the coming days".

Washington swiftly derided the upcoming summit as a sign that Putin was "begging" for help with his 18-month-long anti-Kyiv operations.

The announcement ends days of speculation after US and other officials told The New York Times recently that Kim, who rarely leaves North Korea, was likely to head by armoured train to Vladivostok for arms talks with Putin.

Kim has not travelled outside the North since the start of the coronavirus pandemic.

South Korea's Yonhap news agency quoted an unidentified official as saying "intelligence authorities believe the train presumed to be carrying Kim Jong-un is moving to Vladivostok". 

Broadcaster YTN said Seoul "expects that Chairman Kim will hold a meeting with President Putin of Russia around the day after tomorrow", meaning Wednesday.

Moscow, 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 75 years ago.

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

In July, Putin hailed Pyongyang's "firm support for special military operations against Ukraine".

Vladivostok will be hosting the Eastern Economic Forum until Wednesday.

- 'Pay a price' - 

The White House recently warned that Pyongyang would "pay a price" if it supplies Moscow with weaponry for its war in Ukraine.

On Monday the United States described Putin as desperate over the conflict 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 told reporters.

"President Putin launched this war against Ukraine, with its full-scale aggression, with a dream of restoring the glory of the Russian Empire. That hope, that expectation of his, has failed," Miller said.

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".

Miller renewed US warnings that a Moscow weapons deal with Pyongyang could trigger sanctions, noting: "any transfer of arms from North Korea to Russia would be in violation of multiple United Nations Security Council resolutions."

Andrei Lankov, a North Korea expert at Kookmin University in Seoul, told AFP that a Putin-Kim summit 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.

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".

Kim has become well-known for his preference for train travel when it comes to international trips. His father and predecessor, Kim Jong Il, was famously scared of flying.

In 2019, the current leader made the 60-hour return train trip from Hanoi to Pyongyang by train after a summit with then-US president Donald Trump collapsed, and reportedly hinted at physical fatigue from spending hours on the rails.

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