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 United States is concerned by Russia's growing relationship with North Korea and by what technology the two countries may be exchanging, the top U.S. arms control official said on Wednesday.

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,