That's about as far as Obama is prepared to go so far. He has just stated that the US is sure the aircraft was downed by a SAM from rebel-held territory. They are not confirming who fired, from which system, or why, at the moment.
He has also just stated that the US knows the Russians are supporting the rebels with sophisticated weapons and training (and that further sanctions can be expected as a result). He is not prepared to connect the two statements at the moment.