Russian president Vladimir Putin hailed North Korean troops sent to fight in Ukraine as “heroic” in a letter to Kim Jong-un, North Korean state media reported on Friday.

In a letter marking the anniversary of Korea’s liberation from Japanese rule, Putin recalled how Soviet Red Army units and North Korean forces fought together to end Japan’s colonial occupation.

“The bonds of militant friendship, goodwill and mutual aid which were consolidated in the days of the war long ago remain solid and reliable even today,” Putin said in the letter revealed by North Korean state media.

“This was fully proved by the heroic participation of the DPRK soldiers in liberating the territory of Kursk region from the Ukrainian occupationists,” he said, according to news agency KCNA, using the official name for North Korea.

Earlier this year, Russian forces were engaged in a counteroffensive to eliminate the last of Kyiv’s positions in Russia’s Kursk region, in which Ukraine established a foothold last year.

Russia and North Korea have been forging increasingly close ties, with the two countries signing a mutual defence pact last year, when Putin visited the reclusive state. In April, North Korea confirmed for the first time that it had deployed a contingent of its soldiers to the frontline in Ukraine, alongside Russian troops.

South Korean and western intelligence agencies have said Pyongyang sent more than 10,000 soldiers to Russia’s Kursk region in 2024, along with artillery shells, missiles and long-range rocket systems.

About 600 North Korean soldiers have been killed and thousands more wounded fighting for Russia, Seoul has said. South Korea’s intelligence agency said in June that North Korea was potentially ready to send more troops to Russia.

The letter was delivered by the chair of Russia’s Duma – Vyacheslav Volodin – who met Kim during an official visit to Pyongyang on Thursday.

Volodin thanked Kim for “dispatching excellent soldiers to the Kursk liberation operations for driving out the Ukrainian aggressors”, according to KCNA.

He added that Russia would never forget the North Korean troops “who fought at the cost of their lives in Russia”.

Putin held a phone call with Kim on Tuesday and updated him on planned talks with US president Donald Trump in Alaska on Friday, according to Russian officials.

With Reuters and Agence France-Presse