The Independent reports that Russia's military buildup on the Ukraine border could turn into an invasion on thin pretexts.
There are plenty of Russian troops already inside Ukraine (without even counting Crimea). I wouldn't be surprised if their massive troop buildup is just cover for men and materiel to flow into the occupied zones. It's a giant reserve and supply pool for the insurgents.
Most alarming is the Russian air combat exercise coming up. They will of course shoot down or make intercepts on a few Ukrainian planes, maybe drop supplies or do medium-altitude bombing runs. Close air support is probably out because it would be too easy to get photo and video evidence from the ground. But the idea that they will just fly around and pretend to shoot at each other seems far-fetched.
The leaders of the rebellion are already openly identifying themselves as infiltrated Russian citizens, which makes the use of the word "separatist" dubious. The Russian-speakers who actually live in Ukraine are as likely to be terrified of the rebels as they are to support them.