The US and their main agent in Kiev, Nalivaichenko, immediately and correctly understood the threat: not only did this convoy bring much needed humanitarian aid to Lugansk, it also provided a fantastic political and legal "cover" for future Russian actions inside Novorussia. And by "actions" I don't necessarily mean military actions, although that is now clearly and officially possible. I also mean legal actions such as recognizing Novorussia. From their point of view, Obama, Poroshenko, Nalivaichenko are absolutely correct to be enraged, because I bet you that the timing, context and manner in which Russia moved into Novorussia will not result in further sanctions or political consequences. Russia has now officially declared the Ukie national sovereignty as "over" and the EU will probably not do anything meaningful about it.
That, by itself, is a nightmare for Uncle Sam.
Furthermore, I expect the Russian to act with a great deal of restraint. It would be stupid for them to say "okay, now that we violated the territorial integrity of the Ukraine and ignored its sovereignty we might as well bomb the junta forces and move our troops in". I am quite confident that they will not do that. Yet. For the Russian side, the best thing to do now is to wait. First, the convoy will really help. Second, it will become a headache for the Ukies (bombing this convey would not look very good). Third, this convoy will buy enough time for the situation to become far clearer. What am I referring to here?