North Korea's Kim Jong Un boards train bound for Russia

North Korean leader Kim Jong Un appears to have departed aboard a special train bound for Russia and a summit with President Vladimir Putin, South Korean media reported on Monday, citing unnamed senior government sources.

Russia's Interfax news agency reported earlier on Monday that Kim, who rarely travels abroad, was expected to visit Russia's far east "in the coming days".

The Kremlin said on Saturday Putin would be in the far eastern city of Vladivostok to attend the Eastern Economic Forum, which opened on Sunday, but said it had "nothing to say" about a possible visit by Kim.

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".

Kim's last trip abroad in 2019 was also to Vladivostok for his first summit with Putin after the collapse of North Korea's nuclear disarmament talks with former US President Donald Trump.

South Korea's Chosun Ilbo newspaper said Kim left Pyongyang late on Sunday and would meet Putin as early as Tuesday.

The broadcaster YTN reported the special train would take Kim to North Korea's northeastern border with Russia and the summit was likely to be on Tuesday, though it initially reported it would be on Wednesday. Both reports cited unnamed senior South Korean government sources.

Japanese media reported security was being stepped up and refurbishment was taking place at the main train station in the Russian border city of Khasan, where Kim is expected to enter Russia.

Kim does not travel abroad often and, when he does, it is often shrouded in secrecy and security. North Korea's state media have made no mention of any plan by Kim to visit Russia.

South Korea's foreign ministry said it was monitoring developments but declined to confirm any details. Officials at South Korea's unification ministry said they had no information to provide, while officials at the National Intelligence Service could not be reached.

A summit between Kim and Putin would likely focus on military cooperation and possibly a deal to supply arms, U.S. and South Korean officials and analysts have said.

The US has said it would be a "huge mistake" for North Korea to supply Russia with weapons to use in Ukraine and warned Pyongyang would "pay a price".

The US has said talks for North Korea to supply more weapons to Russia are advancing actively despite repeated denials from Pyongyang and Moscow.

The US and South Korea have expressed concern over the possibility of greater military cooperation between the North and Russia and a potential arms deal.