Aggression from one cat to the other...

My little black cat 'salem' has taken to beating up my other cat 'tabatha.'

Tabatha is older by about four months but is actually a lot smaller in size due to her being the runt of her litter. Salem has just recently started attacking her for no reason, then when I try and separate them he turns on me.

I dont know what to do.... can anyone offer any advice?
thanks