Analysts have said that North Korean artillery shells are poor quality but are essential to the Kremlin because there are so many of them.

The shells helped Russia plug a shortage in the second half of last year and allowed its forces to push forward using costly mass infantry tactics over the past 12 months.

The US-based Institute for the Study of War said North Korea had delivered 4.8 million artillery shells to Russia by June, deliveries which it said had “enabled Russia to maintain a significant artillery advantage over Ukraine”, which has struggled to maintain supply lines.

In September, reports from Ukraine said North Korean missiles had also been fired at Ukrainian troops. There have also been reports from Russia that South Korean labour may be imported to fill gaps in Moscow’s labour force, created by the war in Ukraine.

Lim Eul-chul, a professor at the Institute for Far Eastern Studies in Seoul, said that after sending Russia weapons, it was a logical next step for Kim to send his soldiers to Ukraine.

“It’s crucial for North Korea to learn how to handle different weapons and gain real-world combat experience,” he said.

Kim, who inherited control of North Korea when his father died in 2011, has become such an important ally for the Kremlin that Putin dispatched Sergei Shoigu, the secretary of his security council, to Pyongyang in mid-September for “unspecified bilateral discussion”.

This fondness appears to be reciprocated by Kim. On Monday, he sent his personal congratulations to Putin for his 72nd birthday, calling him his “closest comrade”.