Again, evidence is not proof.

Evidence is put forward and subjected to scrutiny to see if it is valid (or, reasonably considered as such). It is then used with other evidence to prove a case - with some pre-defined level of confidence.
A statements alone is not evidence, no matter where they come from, Russia, USA, France, Syria, Rebels, or elsewhere. When they make statements, one should ask, where is your evidence to back that statement up? Producing evidence long after a statement is made frequently fuels suspicion, as it did with the USA and likely does with others - yes, even Russia.