North Korea’s Kim Jong Un reportedly arrived in Russia Tuesday ahead of an expected meeting with President Vladimir Putin, officials said.

The two leaders, flanked by their top military officials, are set to negotiate a potential arms deal to help resupply Moscow’s military drained by its war in Ukraine. North Korea may have tens of millions of artillery shells and rockets that could give a boost to the Russian army, analysts say.

North Korea’s official Korean Central News Agency previously said Kim boarded his personal train Sunday afternoon en route to Russia, accompanied by unspecified members of the country’s ruling party, government and military.

Jeon Ha Gyu, spokesperson of South Korea’s Defense Ministry, said South Korea's military confirmed the train crossed into Russia sometime early Tuesday morning.

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Kim Jong Un waving

This Sept. 10, 2023 photo provided by the North Korean government shows that North Korea leader Kim Jong Un waves from a train in Pyongyang, North Korea, as he leaves for Russia. (Korean Central News Agency/Korea News Service via AP)

South Korean officials have been unable to identify Kim’s delegation, although it likely includes his foreign minister, Choe Sun Hui, and his top two military officials – Korean People’s Army Marshals Ri Pyong Chol and Pak Jong Chon.

Other officials identified in North Korean state media photos include Pak Thae Song, chairman of North Korea’s space science and technology committee, and Navy Adm. Kim Myong Sik, who have both been a part of North Korea’s efforts to acquire spy satellites and nuclear-capable ballistic missile submarines.

A photo of Kim's train

A green train with yellow trimmings, resembling one used by North Korean leader Kim Jong Un on his previous travels, is seen steaming by a slogan which reads "Towards a new victory" on the North Korea border with Russia and China seen from China's Yiyanwang Three Kingdoms viewing platform in Fangchuan in northeastern China's Jilin province on Monday, Sept. 11, 2023.  (AP Photo/Ng Han Guan)

Train tracks

Russia and North Korea confirmed Monday that North Korean leader Kim Jong Un will visit Russia in a highly anticipated meeting with President Vladimir Putin that has sparked Western concerns about a potential arms deal for Moscow's war in Ukraine.  (AP Photo)

Experts have said North Korea requires external help to achieve these technologies, although it's not clear if Russia would be willing to share.

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Kim Jong Un is also apparently bringing Jo Chun Ryong, a ruling party official in charge of munitions policies who also joined the dictator on recent tours to factories producing artillery shells and missiles, according to South Korea’s Unification Ministry, which analyzed the North Korean photos.

Kim and Putin

Russian President Vladimir Putin, center right, and North Korea's leader Kim Jong Un shake hands during their meeting in Vladivostok, Russia, Thursday, April 25, 2019.  (Yuri Kadobnov/Pool Photo via AP)

Kim, other officials at a train station

This Sunday, Sept. 10, 2023, photo provided by the North Korean government, shows that North Korea leader Kim Jong Un greets attendants in Pyongyang, North Korea, before boarding on a train to Russia.  (Korean Central News Agency/Korea News Service via AP)

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Putin was last seen in the eastern Russian city of Vladivostok, where he attended an international forum on Monday, according to Russia’s TASS news agency. Kremlin spokesperson Dmitry Peskov has said Putin and Kim will meet after the Vladivostok forum, which ends Wednesday, but did not specify when or where.

U.S. officials are monitoring the international meeting between the heavily sanctioned countries and have vowed additional sanctions, should the meeting produce an arms agreement in violation of international security resolutions.

"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, referencing North Korea’s official name. "We urge the DPRK to abide by the public commitments that Pyongyang has made to not provide or sell arms to Russia."

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State Department spokesman Matthew Miller added: "Any transfer of arms from North Korea to Russia would be a violation of multiple U.N. Security Council resolutions," and the U.S. "will not hesitate to impose new sanctions."

Putin’s first meeting with Kim in 2019 was held in Vladivostok. The city is about 425 miles north of Pyongyang.

The Associated Press contributed to this report.