Wouldn't be very difficult in either case to determine who the culprit was with enough intelligence and with the evidence of the blast itself, i.e. its size, the type of fissile material used, and so on; there's also a very limited list of nations that have such weapons in the first place. Assuming in either situation it was Iran or NK it still wouldn't make any sense for them to start such a war they have no means of finishing, or winning.
Iran still doesn't have a nuclear weapon. They're not claiming to, and last I heard the intelligence was they lacked enough fissile material (so far). They haven't tested one either (no country is considered to have them until they successfully test them).
I'm telling you, some people can't differentiate reality from a G.I. Joe cartoon subplot.