From his speech today:
We'd have to cut off countries like China from the American financial system. And since they happen to be major purchasers of our debt, such actions could trigger severe disruptions in our own economy, and, by way, raise questions internationally about the dollar's role as the world's reserve currency. That's part of the reason why many of the previous unilateral sanctions were waived.
There you have it. This has nothing to do with nuclear proliferation. It has everything to do with maintaining the Petrodollar hegemony.