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.

Thousands of North Korean troops helped Russia regain some of its territory in the Kursk region following Ukraine's counterattack, but the Russian army is now using them less on the front line and have pulled some back.

South Korea's spy agency says that North Korean forces have suffered more than 3,000 casualties while fighting alongside Russian troops.