We're staying away from no-fly zones because we consider that it might lead to a direct confrontation between US and Russia - and that might be an "act of war".

OK, that's using our definition of what might be an act of war. Putin seems to be using different definitions. For instance, sanctions are an act of war, any Ukranian military flights originating in a NATO country are an act of war by that country.

But then, he's been at war with the West since he came to power - it's just that the West hasn't been at war with Russia.