I think it's a Weasel. Difficult to judge size on photo, but Stoat has a very clear demarcation between the white and brown fur on the sides, while Weasel has a "messy" transition, as here. Weasels also often have "jugular spots" of brown fur on the chin, which I think I can see on the animal's right side here; Stoats don't have these. Finally, Stoats tend to have more prominent ears, the ears here look more like Weasel.
ETA - great shot, I don't think I've ever seen one that wasn't streaking away like a hairy piece of string!