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.

"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.