More likely a bomb or mechanical failure.
If isis actually did it as they claim look out below. The russians will hunt them down and destroy them. There will not be any strongly worded response.
We are what we experienced when. Putin was a young man during the years of the Afghanistan war, he was 27 when it started and 37 when the Berlin wall came down. Just like the legacy of Vietnam shaped some of us, like Schwarzkopf and Tommy Franks who swore never again, the events of Putins early adulthood shaped him. He will not lose a war with isis. That is my bet.
When the one you piss off happens to have nuclear weapons and a history of brutal response when attacked, they might indeed want to care about who they piss off.