How about this then...
You can't prove that they haven't bought/made a nuclear warhead from other countries. At the same time, you can't say that they have.
So in that way, you have to claim that they both have and don't have a weapon capable of flattering a whole city. It's bascly a Schrodinger's box, in a way, that you have to hope for the best, but prepair for the worse!
(Either way, the only way to prove that i'm wrong is to peek inside the box, that means to prove that they defenetly don't own it, which at the current regime is nearly impossible...)