South Korea's spy agency says North Korea appears to have sent more troops to Russia, after its soldiers deployed on the Russian-Ukraine fronts suffered heavy casualties.

Ukrainian troops holding positions in Kursk say the North Koreans have reappeared on the battlefield and become even more formidable foes.

The fresh deployment comes after North Korea suffered heavy casualties on the front lines in Russia’s war against Ukraine, the South’s spy agency says.

In addition to the contingent that is fighting alongside the Russian forces, Pyongyang has sent millions of artillery munitions and other security assistance to Moscow.

Two North Korean soldiers captured in January in the Kursk region are currently receiving medical treatment. They are testifying about North Korea's involvement in the war on Russia's side, The Wall Street Journal reports.