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.

South Korea's Defence Ministry spokesman said it believes Kim entered Russia early Tuesday.

Kim does not travel abroad frequently, making just seven trips away from his country and twice stepping across the inter-Korean border in his 12 years in power. Four of those trips were to the North's main political ally, China.

"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 reports of Kim's arrival.

US 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.

Putin arrived in Vladivostok on Monday, Russia's TASS news agency said. He is scheduled to attend the plenary session of the Eastern Economic Forum, which runs through Wednesday.

Peskov said that his meeting with Kim would come after the forum and that no news conference by the leaders is planned, according to Russia news agencies.

There has been no confirmation of the location of the meeting or whether Kim would attend the economic forum.

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.

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 on his last trip abroad.

Peskov said Russia's national interests would dictate its policies, according to Russian news agencies.

"As you know, while implementing our relations with our neighbours, including North Korea, the interests of our two countries are important to us, and not warnings from Washington," Peskov was quoted as saying.

Delegation of defence officials

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.

South Korean Vice Foreign Minister Chang Ho-jin, the former ambassador to Russia, said it would be in Moscow's interest to consider its international standing after the Ukraine conflict and remember that it helped form the current nonproliferation regime.

"Military cooperation would be violating Security Council resolutions, whatever (Russia) does with the North," he said.

On Monday, Washington renewed its warnings to Pyongyang not to sell arms to Russia that could be used in the Ukraine war, urging the North to abide by its promise not to provide or sell weapons to Russia.

The US State Department said any transfer of arms from North Korea to Russia would violate UN 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 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, 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 Security Council resolutions as late as 2017 punishing North Korea for ballistic missile launches and nuclear tests.