In the message, received in Pyongyang last week, Putin praised the participation of North Korean soldiers aiding Moscow's war effort against Ukraine, saying it had proved a "militant fraternity" between the two nations.

South Korean and Western intelligence agencies have estimated that the North sent more than 10,000 soldiers to Russia in 2024 -- primarily to the Kursk region -- along with artillery shells, missiles and long-range rocket systems.

"The heroic entry of soldiers of the Korean People's Army into the battles for liberating the Kursk region from occupiers and the subsequent activities of Korean engineers in the land of Russia clearly proved the invincible friendship," Putin wrote, according to the official KCNA news agency.

He added that the provisions of a "historic treaty on comprehensive strategic partnership" signed during his visit to Pyongyang in June last year had been fulfilled "thanks to our joint efforts".

The pact includes a mutual defence clause pledging immediate military assistance if either country faces armed aggression.

Strong Pyongyang-Moscow ties will "contribute to establishing a just order of the multi-polar world", Putin said.

Seoul's spy agency has estimated that around 2,000 North Korean soldiers deployed to help Russia have been killed.

North Korea only confirmed in April that it had deployed troops to support Russia's war in Ukraine and admitted that its soldiers had been killed in combat.

Since then, Kim has met with the families of soldiers killed fighting for Russia against Ukraine and offered condolences for their "unbearable pain".

State media has published images of an emotional Kim embracing a returned soldier who appeared overwhelmed, burying his face in the leader's chest.

After the Moscow-Pyongyang military deal last year, Seoul -- which was under the conservative Yoon Suk Yeol administration then -- said it could reconsider supplying Kyiv with arms, despite a domestic policy banning such a move. But this has not materialised.