SEOUL: North Korean leader Kim Jong Un pledged his "invariable support" for Russia in a birthday letter to President Vladimir Putin, while hailing Moscow's war in Ukraine as "sacred", state media reported on Wednesday (Oct 7).

Since Russia launched its invasion of Ukraine in 2022, North Korea has become a key ally of the Kremlin, sending troops to support Moscow's war effort in return for economic benefits and military technology.

South Korea's spy agency estimated last year that around 2,000 North Korean troops had been killed in the war, while a more recent assessment put the number killed or wounded at around 6,000.

"I am pleased to have good friendship with you and open up a new chapter for the development of the relations between our two countries," Kim said in the letter, according to the official Korean Central News Agency. "I reaffirm my invariable support for you and the Russian Federation," said Kim, adding that he is "firmly convinced that the Russian people's sacred war of justice ... will surely end with their victory".

The letter comes as the South Korean government's relationship with Ukraine has been strained, after President Volodymyr Zelenskyy revealed that two captured North Korean soldiers had been transferred to Seoul's custody. Seoul has condemned the disclosure, saying the operation was meant to be confidential, complicating Kyiv's requests for armaments supplies.

South Korea has so far not provided lethal weapons to Ukraine, citing a domestic ban on militarily aiding countries at war. Zelenskyy has suggested that Pyongyang is poised to send more troops to Russia to aid its war effort against Ukraine.