This is definatly a stage, in my opinion i would leave her, but i do practice controlled crying, if you cant do this then Im sure there are lots of other people who have more info.
I think this is a seperation anxiety, what happened if you stay in the room but just sit on a chair or down by the cot and hold her hand? would she go to sleep this way or would she just play?.
Other than that, have you tried getting her a night light or something that shines light or pictures on the cealing?. We had one of these when my son was little, Its a whinnie the poo one and you wind the thing on the side, it hangs in their cot and they can see the dim picture moving around and it also shines it on the roof while playing music. Mayb something like this will help distract her till she falls asleep ??. HTH