You've used 2L in 1500KM (~1000miles), that is very high. I had consumption similar to that once.....had a hole in the top of a piston!
If it's not leaking, then it is being consumed - either bottom end (piston rings) or top end (valve stem seals). I would say at those levels you're probably looking at piston rings, but a leak down test will tell you. At the very minimum you could do a compression test, but that only tells you that you have a problem, not where the problem is.