Terms such as civil war or internal conflict are often used to describe the war in Eastern Ukraine. However, the available evidence no longer supports this view. During summer 2014, Ukrainian officials and also the U.S. government were already publicly stating that the Russian military were active in the war. A number of subsequent reports have corroborated these claims, documenting the presence and death of Russian servicemen in Eastern Ukraine, as well as the existence of Russian military equipment inside Ukraine.
https://www.bellingcat.com/news/uk-and-europe/2016/12/21/russian-artillery-strikes-against-ukraine/