Seoul is worried that Pyongyang’s price for helping Moscow could be money, military equipment or know-how that it would then use against South Korea.

Seoul has voiced “grave concern” about their pact and called on Moscow to stop its “illegal co-operation” with Pyongyang.

There have been reports that South Korea will deploy military intelligence troops to Ukraine to analyse North Korean tactics and interrogate prisoners.

South Korea is a major arms exporter, though it has a policy of not providing weapons to countries already at war. North Korea’s deployment could put pressure on the South Korean government to try to overcome that.

A proxy war on Ukraine’s territory

If South Korea becomes more heavily involved, the two countries may in effect find themselves fighting a proxy war on Ukraine’s territory.

Darcie Draudt-Véjares, a fellow with the Carnegie Endowment, said: “This potential deployment of third-party ground forces risks transforming the conflict between Russia and Ukraine into even more of a global security crisis, with particularly stark implications for the Korean Peninsula.”

North Korea’s support for Russia has evolved over time.

Kim first supplied ammunition, including millions of artillery shells and missiles which have been used on Ukraine’s cities. There have also been reports of technical advisers on the ground.

Sending thousands of troops marks a sea change however, and analysts believe that presumably comes with a far higher price tag for Russia.

How the new alliance between Russia and North Korea affects global security depends on what that price was and what concessions Kim might have obtained from the Kremlin.

There are several things that Kim may want from Russia, analysts say, and his price may have included a whole package.

Firstly, there is a simple financial motive. Kim is desperate for foreign currency to keep his regime afloat and North Korea has long sent workers to Russia to earn money.