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.

A North Korean soldier captured fighting for Russia in Ukraine last month reportedly made just two requests to his captors - one for water and another for a cigarette

One Ukrainian special forces soldier shares details of how his team captured the North Korean, giving an insight into the North Korean mindset.