North Korean military engineers have been sent to help Russia attack Ukraine with ballistic missiles, and fighters operating in occupied areas of the country have already been killed, senior officials in Kiev and Seoul said.

Behind Russian lines there are dozens of North Koreans in teams “supporting launch systems for KN-23 missiles,” a source in Ukraine told the Guardian.

Kim Jong-un, the North Korean leader, traveled to Russia last year for a summit with Vladimir Putin, where the two men cemented their growing ties through a secret arms deal.

Pyongyang's munitions shipments were crucial to enabling Russian forces to advance in a grueling war of attrition in eastern Ukraine this summer. However, it seems increasingly clear that the agreement went beyond the supply of materials.

South Korean and Ukrainian officials said North Koreans were among the dead after a Ukrainian missile attack on Russian-occupied territory near Donetsk last week. It was not clear whether they were military engineers or other forces.

Foreigners have fought for Russia as mercenaries, but if North Koreans are on the ground it would be the first time a foreign government has sent troops in uniform to support Moscow's war.

South Korean Defense Minister Kim Yong-hyun told lawmakers in Seoul this week that it was “highly likely” that North Korean officers had been deployed to fight alongside the Russians and that several had died in the attack, although he gave no further details .

Andriy Kovalenko, the head of Ukraine's Center for Countering Disinformation, said in a post on Telegram that some North Koreans had been killed in Russia. His organization is part of the National Security and Defense Council.

On Wednesday, the Ukrainian military said it had destroyed North Korean ammunition in an attack on a depot in the Bryansk region, 75 miles (120 km) from the Ukrainian border.

Participation in the war against Ukraine gives North Korea a chance to test weapons, gain combat experience for its troops and strengthen its position with a powerful international ally.

“It is crucial for North Korea, which has supplied Russia with numerous shells and missiles, to learn how to use various weapons and gain practical combat experience,” said Lim Eul-chul, a professor at the Institute of Far Eastern Studies in Seoul. said the AFP news agency. “This could even be a driving factor for sending North Korean soldiers – to give them diverse experiences and war training.”

North Korean missiles and shells are poor quality and unreliable, but were key to Russian weapons firing relentlessly at Ukraine's better trained and motivated army.

Pyongyang is estimated to have supplied about half of the large-caliber ammunition used on the battlefield this year, more than two million rounds, a Ukrainian source said. It also provided KN-23 missiles, which were used in dozens of attacks across Ukraine last winter, Ukrainian media reported. After a break of several months, they were used again from July.

The KN-23 is a short-range ballistic missile that was first tested in 2019 and compared to Russia's Iskander-M missiles. The range is estimated to be around 280 miles with a 500kg warhead.

Moscow and Pyongyang deny arms sales, although they have publicly celebrated deepening ties in recent months. The Kremlin on Thursday dismissed the stationing of North Korean troops in Ukraine as “another hoax.”

Kim called Putin his “closest comrade” in a birthday message sent this week, and Putin made a state visit to North Korea in June, during which the leaders signed a mutual aid agreement.

In exchange for its missiles and other military equipment, North Korea is believed to be looking for Russian help for its spy satellite program, which has suffered embarrassing failures over the past two years.

It is not clear how far Russia is willing to share sensitive military technology with North Korea in return for continued support in Ukraine.

Pyongyang is seeking to strengthen its ties with Russia and China as part of an alliance against “Western hegemony and imperialism” after decades of U.N. sanctions over its ballistic missile and nuclear weapons programs.

The strategy paid off in March when Russia used its UN Security Council veto to effectively end UN monitoring of sanctions violations, a move publicly welcomed by Pyongyang.

Artem Mazhulin contributed to this to this report.