A TV screen shows an image of North Korean leader Kim Jong-un's departure from Pyongyang, North Korea for Russia, during a news program at the Seoul Railway Station in Seoul, South Korea, Tuesday, Sept. 12. AP-Yonhap

North Korean leader Kim Jong-un has arrived in Russia, Japanese media reported on Tuesday, for what the Kremlin said would be a comprehensive discussion with President Vladimir Putin amid warnings from Washington they should not agree on an arms deal.

Kim left Pyongyang for Russia on Sunday on his private train, the North's state media reported on Tuesday, accompanied by top arms industry and military officials and the foreign minister.

Japan's Kyodo news agency reported on Tuesday, citing an unnamed Russian official source, that a train carrying Kim had arrived at Khasan station, the main rail gateway to Russia's Far East from North Korea.

"It will be a full-fledged visit," Kremlin spokesman Dmitry Peskov said. "There will be negotiations between two delegations, and after that, if necessary, the leaders will continue their communication in a one-on-one format."

An official at the Khasan administration declined to comment on the report of Kim's arrival.

Putin was expected to arrive in Vladivostok on Tuesday, where he was scheduled to attend the main session of the Eastern Economic Forum, which runs through Wednesday.

His meetings with Kim were expected to be on the sidelines of the forum, although there has been no confirmation of the location or whether Kim would attend the event.

U.S. officials, who first said the visit was imminent, said that arms talks between Russia and North Korea were actively advancing and that Kim and Putin are likely to discuss providing Russia with weapons for the war in Ukraine.

Pyongyang and Moscow have denied that North Korea would supply arms to Russia, which has expended vast stocks of weapons in more than 18 months of war.

North Korea's KCNA state news agency said Kim was accompanied by leading officials of the ruling Workers' Party, government and armed forces.

Washington and its allies have been voicing concern at recent signs of closer military cooperation between Russia and the nuclear-armed North. It will be Kim's second summit with Putin, after they met in 2019.

This photo, carried by North Korea's official Korean Central News Agency on Sept. 12, shows the North's leader Kim Jong-un leaving Pyongyang for a trip to Russia on his special train. Yonhap

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The North Korean delegation includes prominent members of the party who handle defence industry and military affairs, including Munitions Industry Department Director Jo Chun Ryong, an analyst said, which suggests the visit will focus on defence industry cooperation.

"The presence of Jo Chun-ryong indicates that North Korea and Russia will conclude some type of agreement for munitions purchases," said Michael Madden, a North Korea leadership expert at the Washington-based Stimson Center.

Photos released by state media showed military honour guards and crowds of people in dark suits and colourful dresses waving flowers and flags as Kim boarded the dark-green train, which is believed to be armoured and carry other specialised equipment.

On Monday, as the train was bound for the Russian border, Washington renewed its warnings to Pyongyang not to sell arms to Russia that could be used in the Ukraine war.

"We urge the DPRK to abide by the public commitments that Pyongyang has made to not provide or sell arms to Russia," said Adrienne Watson, spokesperson for the White House National Security Council.

DPRK is short for the North's official name, the Democratic People's Republic of Korea.

The U.S. State Department said any transfer of arms from North Korea to Russia would violate multiple U.N. Security Council resolutions, which ban any arms transactions with North Korea.

"We, of course, have aggressively enforced our sanctions against entities that fund Russia's war effort, and we will continue to enforce those sanctions and will not hesitate to impose new sanctions appropriately," State Department spokesperson Matthew Miller told reporters.

North Korea is one of the few countries to have openly supported Russia since the invasion of Ukraine last year, and Putin pledged last week to "expand bilateral ties in all respects in a planned way by pooling efforts".

In a striking display of those deepening ties, Kim gave a personal tour of an arms exhibition for Russian Defence Minister Sergei Shoigu when he visited Pyongyang in July, and they stood together to watch a military parade that featured banned ballistic missiles.

Russia had voted, along with China, to approve U.N. Security Council resolutions as late as 2017 punishing North Korea for launching ballistic missiles and conducting underground nuclear tests.

Peskov said the main topic of the talks by Putin and Kim would be bilateral relations between the neighbouring countries.

"We will continue to strengthen our friendship," Peskov said. (Reuters)