Well to be accurate, ATP is the ONLY fuel source for muscular action. Creatine, Fat or glycogen can be used for energy, but they do so by producing more ATP. Ingesting creatine doesn't increase ATP stores. However, once your ATP is depleted (it becomes ADP/adenosine diphosphate + P + energy) creatine will give its P molecule to the ADP to reform more ATP.
There is enough ATP reserve in your muscles for maybe 3-5 seconds of intense work, you really cannot stock more than that. After that point you'll have to rely on creatine, glycogen and fat for energy ... they will do so by producing more ATP that can be used for muscle contraction.