The Russians are an odd lot. The separatists are recruited in Russia, trained in Russia, and supplied from Russia (even paid, in some cases), but the Russians feel that it is important to deny that they are regular Russian troops.
Glad the US has never done anything like that. Regular Russian forces aren't involved in the fighting -- or if they are, it's been to a very limited extent. I can speculate about why that may be the case, but I think the fact is self evident.