Last month, the US revised its guidance on its Iran sanctions (link here) to state that:
1) Transactions between foreign entities and non-sanctioned entities that are minority owned, or controlled in part by an Iranian or Iran-related SDN are “not necessarily sanctionable” under US law. OFAC recommends caution in respect of those transactions, to ensure that they do not directly involve a sanctioned person.
2) Screening the name of a potential Iranian counterparty against the SDN list is an expected but not necessarily sufficient due diligence step, and non-US persons should consult their local regulators and maintain records documenting due diligence efforts.
3) OFAC advises non-US persons that OFAC does not expect non-US financial institutions to repeat the due diligence their customers have performed on Iranian customers, unless they believe that those processes were insufficient.
Secretary of State John Kerry made a statement suggesting that ordinary due diligence was now acceptable with respect to Iran. A State Department official has since then made clear that banks and businesses are expected to exercise due diligence in any overseas investment or transaction, and for a high-risk jurisdiction like Iran the norm is enhanced due diligence.