North Korea's leader Kim Jong Un appears to have started his journey to Vladivostok for a summit with Russia's President Vladimir Putin.
The armoured train that Mr Kim uses for foreign visits appears to have departed, South Korean media reported citing a government official.
The meeting is expected to take place as early as Tuesday local time.
Russian news agency Interfax has reported earlier that Mr Kim is expected to visit "in the coming days".
If the summit with Mr Putin pushes through, it will be the North Korean leader's first international trip in more than four years, and the first since the pandemic.
The two leaders will likely discuss the possibility of North Korea providing Moscow with weapons to support its war in Ukraine, a US official earlier told the BBC's US partner CBS.
Mr Kim's last trip abroad was also to Vladivostok in 2019 for his first summit with Putin after the collapse of North Korea's nuclear disarmament talks with then US President Donald Trump.
Mr Kim also travelled to Vladivostok by train in 2019.
It is rumoured to include at least 20 bulletproof cars, making it heavier than average trains and unable to go beyond 59 km/h (37mph). His journey to Vladivostok is expected to take an entire day.
The possible meeting comes after the White House said it had new information that arms negotiations between the two countries were "actively advancing".
National Security Council spokesman John Kirby earlier said Russia's Defence Minister, Sergei Shoigu, had tried to "convince Pyongyang to sell artillery ammunition" to Russia during a recent visit to North Korea.
It is thought that Russia might need 122mm and 152mm shells because its stocks are running low, but it is not easy to determine North Korea's full artillery inventory, given its secretive nature.
Weapons on display at the meeting between Mr Kim and Mr Shoigu in July included the Hwasong intercontinental ballistic missile, believed to be the country's first ICBM to use solid propellants.
It was the first time Mr Kim had opened the country's doors to foreign guests since the Covid pandemic.
