The recently signed mutual defense treaty could be used to justify thewidening of the war in Ukraine, as the U.S. says North Korean and Russian troops are already battling Ukrainian forces.

The North Korean self-propelled guns have a range of between 25 to 37 miles but an "incredibly slow" rate of fire.

A photo has emerged showing examples of a North Korean-made self-propelled artillery system under transport in Russia, the clearest suggestion so far that Pyongyang is also now providing Moscow with these kinds of heavyweight,