Whoever is holding them, any rescue attempt would have to kill a load of Russian/Ukrainians, who's to tell. And if it went wrong there would have to be a much larger invasion to extricate. Suddenly we are fighting someone else's civil war. How can you demand NATO military action in Ukraine while insisting Russian military there are terrorists?
There is of course a principled case, rh, to be made for intervention. But if you're going to make that case, you must then explain why you aren't/weren't in favour of doing so everywhere else in the world (volunteers for Chechnya anyone?)people are doing vile things to each other.