North Korean leader Kim Jong-un has embarked on a train journey on Monday, government sources confirmed to The Korea Herald, strongly indicating the possibility of a forthcoming meeting with President Vladimir Putin in Russia. The move comes amid blistering criticism of talks between the two leaders on arms trade, which has raised concerns about potential support for Russia's invasion of Ukraine.

The armored train, presumably carrying Kim, was slowly advancing toward the northeastern border as of Monday, according to South Korean government officials who wished to remain anonymous due to the sensitivity of the matter.

Both countries have maintained their silence on the matter as of Monday afternoon, even as Putin embarks on his two-day visit to the far eastern port city of Vladivostok on the same day to participate in the 8th Eastern Economic Forum. US intelligence has pointed to the four-day event continuing throughout Wednesday as the venue for the summit.

US intelligence released information about the possible summit schedule, which opens up the possibility that the meeting might be relocated to other dates and places or even Moscow. Last week, the South Korean intelligence agency also said that there is a chance that Kim may make an unforeseen decision and deviate from the expected route.

However, the summit is likely to proceed as originally planned on the sidelines of the Eastern Economic Forum, the source earlier in the day confirmed to The Korea Herald, ruling out the potential of holding the summit in Moscow.

Seoul officials believe that the meeting will take place either on Tuesday or Wednesday with the expectation that Kim will stay in Russia until Wednesday.

This decision is influenced by Putin's challenges in rescheduling the summit amid the ongoing war in Ukraine.

Given that Kremlin spokesperson Dmitry Peskov on Monday dismissed the plan to hold the Kim-Putin summit at the EEF itself during an interview with RTVI, there is potential for the meeting to take place in Vladivostok or another area in the Russian Far East following Putin's participation in the EEF on Tuesday.

While keeping a tight lid on the potential summit, North Korean state media reported Monday that Kim had a photo session with participants in the paramilitary parade celebrating the 75th anniversary of the country's founding on Sept. 9. The event took place Sunday in Kim Il-sung Square in Pyongyang, leaving open the possibility that Kim could depart for Russia afterwards.

The silence stands in stark contrast to the previous summit between Kim and Putin in April 2019.

Kim visited Vladivostok from April 24 to 26 for his first-ever meeting with Putin that year. During this visit, the Kremlin officially disclosed Kim's impending visit six days prior to his arrival, whereas North Korean state media only officially announced the trip just one day before Kim's departure.

Kim Jong-un has employed both his private armored train and planes for overseas summits in the past.

Kim opted to travel by train when meeting with Chinese General Secretary Xi Jinping in March 2018 and again in January 2019. Similarly, he used a train for his visit to Hanoi, Vietnam, to hold his second summit with then-US President Donald Trump in February 2019. Furthermore, in April 2019, Kim Jong-un continued to favor travel by train when he journeyed to Vladivostok for his meeting with Putin.

At that time, Kim commenced his journey on an armored private train, sporting its distinctive dark green color, in the early hours of April 24, departing from North Korea, likely not from Pyongyang. Subsequently, the train carrying Kim arrived at the border station of Khasan at around 10:40 a.m. local time, which corresponds to 9:40 a.m. Pyongyang time, on the same day.

After a one-hour stopover, the train resumed its journey, reaching Ussuriysk Station before making a transition onto the Trans-Siberian Railway, ultimately bound for Vladivostok. Kim arrived at Vladivostok Station at around 6:00 p.m. after an approximately 7-hour trip from Khasan.

Kim was widely expected to depart from Pyongyang no later than Monday as the journey takes 21 hours by train, based on his visit to the Russian city in 2019. The train four years ago traveled 47 kilometers per hour, according to reports.

The potential Kim-Putin summit has garnered significant attention from the US and its allies and partners. The US has detected indications that the primary agenda for the meeting could revolve around North Korea's provision of weapons, including ammunition, to Russia in support of its full-scale invasion of Ukraine.

Russia has also been accused of transferring advanced technologies for weapons, including satellites and nuclear-powered submarines, to North Korea. Kim has directed the development of weapons as part of North Korea's five-year defense plan spanning from 2021 to 2026.

South Korea's Defense Minister Lee Jong-sup acknowledged these reciprocal exchanges between North Korea and Russia, pledging to take countermeasures in coordination with the US, during an interpellation session of the National Assembly on Sept. 6.

"If it is confirmed that discussions have taken place regarding nuclear-powered submarines between Russia and North Korea, we will initiate discussions with the US on this issue."

South Korea's Unification Ministry on Monday said, "The government expresses concerns regarding the raised possibility of military cooperation between North Korea and Russia."

"We once again emphasize that such cooperation between the two countries should not undermine international norms and peace on the Korean Peninsula," ministry spokesperson Koo Byoung-sam told a televised regular briefing.

Both the provision of weapons by North Korea and the transfer of technology related to nuclear and ballistic missile programs by Russia to North Korea are in violation of UNSC resolutions.

US Vice President Kamala Harris emphasized in an interview with CBS News, which aired on Sunday, that the potential Kim-Putin meeting highlights Russia's "desperation" and "strategic failure" amid the grinding war in Ukraine.

Harris also reiterated the Biden administration's clear and publicly stated warning to North Korea regarding repercussions it would face if it were to supply weapons to Russia.

"I think it would be a huge mistake. First of all, when you look at Russia's unprovoked war on Ukraine, and the idea that they would supply ammunition to Russia, well, it's predictable where that ends up," Harris said when asked about the potential meeting.

"I also believe very strongly that for both Russia and North Korea, this will further isolate them."