As I said before, North Korea does not have a nuclear warhead capable of launching on one of their ICBM's. Their ICBM program is early developed. It's simply a baby. Most of their Rockets cannot even complete the 3 stage boost and end up sleeping with the fishies. And not only that, but the US has naval vessels with RIM-161 missile capable of destroying a incoming ICBM's/warheads.
And lets not forget, the US has a large nuclear weapon force, so lets say if one of the missiles were to be launched, all the US has to do is launch their Minuteman III MIRV(multiple independently targetable reentry vehicle) ICBM and/or Trident II ICBM's.
Basically North Korea is starting a fight they simply cannot win.