Here's the question: has the North Koreans reached the point of building an implosion-style nuclear bomb that uses plutonium as the fissile material? If they've reached that point the North Koreans could build warheads with a yield of 45-50 kT for their Nodong-2 missile that could threaten the major US military bases in Japan.
No one has the answer to that question. Looks like we may have to find out the hard way. Their tests have not been spectacular, from what I gather. You would think that if they had that capability, South Korea announcing that they wanted US Tactical Nukes, would not set the North Koreans off like it did. Course that does not guarantee that they are not currently in the act of pursuing those capabilities.
At any rate, those Chinese Carrier Killing ICBM's are the real danger. Their super sonic cruise missiles are also a threat.