In December last year, over 11,000 North Korean troops had been deployed in Russia's Kursk region to aid Russian forces in foiling Ukrainian counterattacks in the ongoing Russia-Ukraine war.

Though North Korean ballistic missiles account for only a small portion of Russia's missile strikes they carry a large warhead of up to one tonne and have a range of up to 800 km, the military source said. The Iskander-M, an equivalent Russian missile, carries a smaller payload and has a shorter range of 500 km.

South Korea’s assessment comes as Ukraine’s stout defence takes a growing toll for every square kilometre Russia seizes.

"The worst problem we are facing is the one coming from North Korea," the head of Kyiv's military intelligence agency said in the fall.