The UN treaty with the United States on “privileges and immunities” states that:
“(1) Paragraph (b) of section 18 regarding immunity from taxation and paragraph (c) of section 18 regarding immunity from national service obligations shall not apply with respect to United States nationals and aliens admitted for permanent residence”.
Thus, there is no direct exception from FBAR reporting for UN accounts.
However, there may be an exception from reporting depending on where the account is held as follows:
“A foreign financial account of any international financial institution (if the United States government is a member) is not required to be reported by any person.”
International financial institutions (IFIs) are financial institutions that have been established (or chartered) by more than one country, and hence are subjects of international law. Their owners or shareholders are generally national governments, although other international institutions (such as the UN) and other organizations occasionally figure as shareholders. The most prominent IFIs are creations of multiple nations, although some bilateral financial institutions (created by two countries) exist and are technically IFIs. Many of these are multilateral development banks (MDB).
However, to qualify the U.S. must be a member of the IFI, such as the World Bank.
To summarize, the key to any exception for UN bank accounts is what bank the account is held in and where that bank is located, and not that it is a UN account.