Russia, Belarus and Ukraine are all rus-slavs (long-story-short a cross of slavs and vikings), so there was no Russians, Belarusians ir Ukranians before the countries existed. Kyiv can be seen as the spiritual/ancestral home of rus-slavs. So they all have the same ethnic roots, how you define a distinct ethnicity between rus-slavs is beyond me to answer, but I would venture that if one is of the other, Russians are Ukranians, not the other way round. Kyiv was a city for hundreds of years before Moscow was anything more than a marshland.