South Korea's defence ministry said it believed that Pyongyang's leader Kim Jong Un entered Russia early on Tuesday on his heavily armoured private train, ahead of a meeting with President Vladimir Putin.

"The Ministry of National Defense believes that North Korea's Kim Jong Un entered Russia early this morning using a private train," it said, after the North's state media and the Kremlin confirmed Kim's trip, his first out of the country in four years.

The North's state media reported on Tuesday that he was 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."

Putin was expected to arrive in Vladivostok – on Russia’s east coast close to North Korea – 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.

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.

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. (AFP/Reuters)