This photo, carried by North Korea's official Korean Central News Agency, Sept. 12, shows the North's leader Kim Jong-un saluted by soldiers ahead of a trip to Russia. Yonhap

North Korean leader Kim Jong-un arrived in Russia early Tuesday, an official at South Korea's defense ministry said, as Kim is set to hold a rare summit with Russian President Vladimir Putin, with concerns growing over a possible arms deal between Pyongyang and Moscow.

North Korea's official Korean Central News Agency (KCNA) confirmed early Tuesday that Kim had left Pyongyang for Russia aboard his train Sunday afternoon, accompanied by leading officials of the North's ruling party and the armed forces.

The KCNA said Kim "left here by his train on Sunday afternoon to visit the Russian Federation," without saying whether the train had crossed its border.

Hours after the KCNA report, Jeon Ha-kyou, a spokesperson at South Korea's defense ministry, gave an assessment that Kim's train had crossed into Russia early Tuesday, adding that the ministry is closely monitoring for possible talks between the two countries over arms trade.

"Considering that a large number of military personnel is accompanying him, (we) are closely monitoring whether negotiations over arms trade between North Korea and Russia, and technology transfers will take place," Jeon told reporters.

Russian media outlet "Vesti Primorye" also reported that Kim's train arrived at the Russian border city of Khasan on Tuesday and is on its way to the Far Eastern city of Ussuriysk, citing a railway source.

The train passed through Khasan station early Tuesday and is already in the Primorsky Krai region, according to the Russian media report.

This photo, carried by North Korea's official Korean Central News Agency, Sept. 12, shows the North's leader Kim Jong-un leaving Pyongyang for a trip to Russia on his special train. Yonhap

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Still, it remains unknown exactly when and where Kim and Putin would hold a meeting, and Kim's whereabouts also remain unclear. The Kremlin said negotiations between North Korean and Russian delegations are planned during Kim's visit, with discussions over the possibility of a one-on-one meeting between the leaders.

It was widely speculated that Kim could travel to Russia's far eastern city of Vladivostok to meet Putin as he previously did in 2019.

Kim's armored train has crossed the railway bridge over the Razdolnaya River in the Primorsky Krai and is moving north, Russian news service RIA Novosti said early Tuesday, raising the possibility that he could first travel to another city before heading to Vladivostok.

The trip would mark the first such visit to Russia by Kim in nearly 4 1/2 years and his first trip abroad since the outbreak of the COVID-19 pandemic.

Pyongyang has recently been seeking to bolster military ties with Moscow in the wake of growing security cooperation among South Korea, the United States and Japan.

The New York Times earlier reported that Kim plans to travel to Vladivostok, possibly by armored train this month, for talks with Putin about the possibility of supplying Russia with ammunition and weaponry for its war in Ukraine and other military cooperation.

As both North Korea and Russia confirmed Kim's visit to Russia, the U.S. called on Pyongyang not to provide any weapons to Russia.

North Korean leader Kim Jong-un, center, arrives at a station in Pyongyang, Sept. 10, before boarding a train to visit Russia to hold talks with Russian President Vladimir Putin, in this photo released by the North's official Korean Central News Agency, Sept. 12. Yonhap

Adrienne Watson, spokesperson for the White House National Security Council, underscored that North Korea and Russia will likely continue discussing a potential arms deal during the Kim-Putin meeting.

"As we have warned publicly, arms discussions between Russia and the DPRK are expected to continue during Kim Jong-un's trip to Russia," Watson told Yonhap News Agency when asked to comment on Kim's visit to Russia.

"We urge the DPRK to abide by the public commitments that Pyongyang has made to not provide or sell arms to Russia," she added.

DPRK stands for the Democratic People's Republic of Korea, North Korea's official name. (Yonhap)