Ukrainian government is not denying Russia buildup on their borders. They just say that Russia does not have enough troops for a full scale invasion. So, Russia is preparing, but is not yet ready for a full-scale invasion.
But USA is indeed going too far in rhetoric if Ukraine is toning them down.
Personally, I think there is 55% chance Russia is bluffing and intimidates Ukraine to dissuade them from thinking about taking back Donbas and Crimea. 45% chance that by end of March there will be land corridor from Russia to Crimea.