It’s a very delicate situation. Seoul is only 40 miles from the DMZ border. China is over the border from NK.
One possibility that South Korea would have to keep in mind is the fleet of ancient biplanes. If sent on a preemptive first strike, it’s likely the first knowledge of such an attack would come with the nuke exploding over Seoul: “The fact that North Korea is now a nuclear nation and seems to have at least a workable miniaturized warhead design means these biplanes pose an even greater risk than ever before—they could also become a non-traditional nuclear delivery system. It is just the unorthodox play that Pyongyang could use to take out major military installations or critical capabilities without using a ballistic missile.“