Pundits have been on TV claiming that making a nuke small and light enough to go on a missile is very difficult and it will take the NORKs years to accomplish it.
I doubt it. The first nuke, the one we used on Hiroshima was so big and heavy that it was all a B29 could do to get off the ground with one on board. By the 1950's we had one small enough to fire out of an 8 inch cannon. Which is small enough to fit on any missile. At least any missile bigger that a 4th of July skyrocket.
The NORKs probably have a little work yet to do before they can nuke the western US, but not much. Secretary Tillerson is saying the right stuff about the NORKs and their nukes.