The 15K figure is probably a conservative estimate. The British have previously said 25K dead at one point.
It doesn't really fit with the 75K casualties figure either. If true, it means a 1:4 KIA to WIA ratio. A 1:3 ratio I believe is the accepted figure when you have good medical care and facilities, and Russia lacks those and has a general indifference to troop wellbeing on top of that. 25k dead and 50k wounded would be a better fit for the Russians. The Ukrainians are now saying that it is passed the 40k dead mark for Russia.
When you consider that one of the separatist groups has admitted to around 3K dead and another 12K wounded (so 15k casualties), you'd have to think the other separatist group would have taken similar losses. No way the Russians have taken only 40k casualties total, which would suggest that it does not include separatist losses.