In a statement posted on the club's official website said: "Russell led the club to the play-offs in his first season and while we had a difficult start to this campaign we maintained complete confidence in him to produce the results we wanted.
"That faith was justified with the run of recent form and the improvement in our league position.
"I would very much like to be in a position to explain the reasons why it was necessary for Russell and the club to part company, but I am unable to do so for legal reasons.
"I am positive that our fans would completely endorse the decision of the board if I was able to share full details of the circumstances. I will make a further announcement as soon as I am able to."