North Korean leader Kim Jong Un has departed for Russia where he is expected to hold a highly anticipated meeting with President Vladimir Putin that has sparked Western concerns about a potential arms deal for Moscow's war in Ukraine.

North Korea's official Korean Central News Agency said on Tuesday that Kim boarded his personal train from the capital, Pyongyang, on Sunday afternoon, and that he will be accompanied by unspecified members of the country's ruling party, government and military.

State media photos showed Kim walking pass honour guards and crowds of civilians holding the national flag and flowers and waving his hand from his green-and-yellow armoured train before it left the station in Pyongyang.

A group of senior officials, including Cabinet Premier Kim Tok Hun, Kim Jong Un's top economic official, were at the station to give the leader a “hearty send-off", KCNA said.

KCNA did not specify whether the train had crossed the border.

A brief statement on the Kremlin's website on Monday said the visit is at Putin's invitation and would take place “in the coming days.” KCNA said the leaders would meet — without specifying when and where.

Kremlin spokesperson Dmitry Peskov said that Putin and Kim will lead their delegations in talks and could also meet “one-on-one if necessary”. He added that Putin will host an official dinner for Kim.

The talks will focus on bilateral ties, Peskov said. “As with any of our neighbours, we feel obliged to develop good, mutually beneficial relations,” he added.

A possible venue is the eastern Russian city of Vladivostok, where Putin arrived Monday to attend an international forum that runs through Wednesday, according to Russia's TASS news agency. The city, located about 425 miles (680 kilometers) north of Pyongyang, was also the site of Putin's first meeting with Kim in 2019.

The visit would be Kim's first foreign trip since the COVID-19 pandemic, which had forced North Korea to enforce tight border controls for more than three years to shield its poor health care system. While Kim has shown to be more comfortable using planes than his famously flight-adverse father, he has also used his personal train for previous meetings with Putin, Chinese President Xi Jinping and former US President Donald Trump, reviving a symbol of his family's dynastic rule.

Associated Press journalists near the North Korea-Russia frontier saw a green train with yellow trim — similar to one used by the reclusive Kim during previous foreign trips — at a station on the North Korean side of a border river.

It was unclear whether Kim was on the train, which was seen moving back and forth between the station and the approach to the bridge that connects the countries. It had not crossed the bridge as of 7 pm (1000 GMT).

Citing unidentified South Korean government sources, the Chosun Ilbo newspaper reported the train likely left the North Korean capital of Pyongyang on Sunday evening and that a Kim-Putin meeting is possible as early as Tuesday.

The Yonhap news agency and some other media published similar reports. Japan's Kyodo news agency cited Russian officials as saying Kim was possibly heading for Russia in his personal train.

South Korea's Presidential Office, Defence Ministry and National Intelligence Service didn't immediately confirm those details.

US officials released intelligence last week that North Korea and Russia were arranging a meeting between their leaders that would take place within this month as they expand their cooperation in the face of deepening confrontations with the United States.

According to US officials, Putin could focus on securing more supplies of North Korean artillery and other ammunition to refill declining reserves as he seeks to defuse a Ukrainian counteroffensive and show that he's capable of grinding out a long war of attrition. That could potentially put more pressure on the US and its partners to pursue negotiations as concerns over a protracted conflict grow despite their huge shipments of advanced weaponry to Ukraine in the past 17 months.

“Arms discussions between Russia and the DPRK are expected to continue during Kim Jong Un's trip to Russia,” said White House National Security Council spokesperson Adrienne Watson, using the abbreviation for North Korea's official name of the Democratic People's Republic of Korea.

“We urge the DPRK to abide by the public commitments that Pyongyang has made to not provide or sell arms to Russia.”

State Department spokesman Matthew Miller said Washington will monitor the meeting closely, reminding both countries that “any transfer of arms from North Korea to Russia would be a violation of multiple UN Security Council resolutions,” and that the US “will not hesitate to impose new sanctions".

North Korea has possibly tens of millions of artillery shells and rockets based on Soviet designs that could potentially give a huge boost to the Russian army, analysts say.

In exchange, Kim could seek badly needed energy and food aid and advanced weapons technologies, including those related to intercontinental ballistic missiles, nuclear-capable ballistic missile submarines and military reconnaissance satellites, analysts say.

There are concerns that potential Russian technology transfers would increase the threat posed by Kim's growing arsenal of nuclear weapons and missiles that are designed to target the US, South Korea, and Japan.

After decades of a complicated, hot-and-cold relationship, Russia and North Korea have been drawing closer since Moscow's invasion of Ukraine in 2022. The bond has been driven by Putin's need for war help and Kim's efforts to boost the visibility of his partnerships with traditional allies Moscow and Beijing as he tries to break out of diplomatic isolation and have North Korea be part of a united front against Washington.

While using the distraction caused by the Ukraine conflict to ramp up its weapons development, North Korea has repeatedly blamed Washington for the crisis in Ukraine, claiming the West's “hegemonic policy” justified a Russian offensive in Ukraine to protect itself.

North Korea is the only nation besides Russia and Syria to recognise the independence of two Russian-backed separatist regions in eastern Ukraine — Donetsk and Luhansk -– and it has also hinted at an interest in sending construction workers to those areas to help with rebuilding efforts.

Russia -– along with China -– have blocked US-led efforts at the UN Security Council to strengthen sanctions on North Korea over its intensifying missile tests while accusing Washington of worsening tensions with Pyongyang by expanding military exercises with South Korea and Japan.

The United States has been accusing North Korea since last year of providing Russia with arms, including artillery shells sold to the Russian mercenary group Wagner. Both Russian and North Korean officials denied such claims. But speculation about the countries' military cooperation grew after Russian Defence Minister Sergei Shoigu made a rare visit to North Korea in July, when Kim invited him to an arms exhibition and a massive military parade in the capital where he showcased ICBMs designed to target the US mainland.

Following that visit, Kim toured North Korea's weapons factories, including a facility producing artillery systems where he urged workers to speed up the development and large-scale production of new kinds of ammunition.

Experts say Kim's visits to the factories likely had a dual goal of encouraging the modernisation of North Korean weaponry and examining artillery and other supplies that could possibly be exported to Russia.

Jon Finer, US President Joe Biden's chief deputy national security adviser, said Sunday that buying weapons from North Korea “may be the best and may be the only option” open to Moscow as it tries to keep its war effort going.

“We have serious concerns about the prospect of North Korea potentially selling weapons, additional weapons, to the Russian military. It is interesting to reflect for a minute on what it says that when Russia goes around the world looking for partners that can help it, it lands on North Korea,” Finer said on a plane carrying Biden from India to Vietnam.

Some analysts say a potential meeting between Kim and Putin would be more about symbolic gains than substantial military cooperation.

Russia, which has always closely guarded its most important weapons technologies, even from key allies such as China, could be unwilling to make major technology transfers with North Korea for what is likely to be limited war supplies transported over a small rail link between the countries, they say.