What? US feel threatened? maybe the whole world threaten by their arms. Any US nuclear tactics or military build-up in east asian region may encounter and made protest by the chinese, because it may cause imbalance the way it would be. The chinese goverment says they will undergo some dialogs between them and the north koreans that is why there will be a reason for the US and the world be threaten. Just leave the chinese do the work as a regional power and keep their toys off in asia or they may cause WW3 if they don't.
as much as i agree with this since since the chances of north korea nuking american mainland is as low as me liking Nicolas cage but you have to know the ones who are really threatened is japan and S.korea the US is just bring in the guns
North Korea currently has no missile technology that would allow a travel distance of that magnitude. Also, Japan and Korea aren't so much threatened as they are worried. they could wipe the floor with the North, but they still don't want a nuke going off.