31 C.F.R. § 594.203 : Money and Finance: Treasury — REGULATIONS RELATING TO MONEY AND FINANCE — OFFICE OF FOREIGN ASSETS CONTROL, DEPARTMENT OF THE TREASURY — GLOBAL TERRORISM SANCTIONS REGULATIONS — PROHIBITIONS — Holding of funds in interest-bearing accounts; investment and reinvestment

(a) Except as provided in paragraph (c) or (d) of this section, or as otherwise directed by the Office of Foreign Assets Control, any U.S. person holding funds, such as currency, bank deposits, or liquidated financial obligations, subject to §594.201(a) shall hold or place such funds in a blocked interest-bearing account located in the United States.
(b)
(1) For purposes of this section, the term blocked interest-bearing account means a blocked account:
(i) In a federally-insured U.S. bank, thrift institution, or credit union, provided the funds are earning interest at rates that are commercially reasonable; or
(ii) With a broker or dealer registered with the Securities and Exchange Commission under the Securities Exchange Act of 1934, provided the funds are invested in a money market fund or in U.S. Treasury bills.
(2) For purposes of this section, a rate is commercially reasonable if it is the rate currently offered to other depositors on deposits or instruments of comparable size and maturity.
(3) Funds held or placed in a blocked account pursuant to this paragraph (b) may not be invested in instruments the maturity of which exceeds 180 days. If interest is credited to a separate blocked account or subaccount, the name of the account party on each account must be the same.
(c) Blocked funds held in instruments the maturity of which exceeds 180 days at the time the funds become subject to §594.201(a) may continue to be held until maturity in the original instrument, provided any interest, earnings, or other proceeds derived therefrom are paid into a blocked interest-bearing account in accordance with paragraph (b) or (d) of this section.
(d) Blocked funds held in accounts or instruments outside the United States at the time the funds become subject to §594.201(a) may continue to be held in the same type of accounts or instruments, provided the funds earn interest at rates that are commercially reasonable.
(e) This section does not create an affirmative obligation for the holder of blocked tangible property, such as chattels or real estate, or of other blocked property, such as debt or equity securities, to sell or liquidate such property at the time the property becomes subject to §594.201(a). However, the Office of Foreign Assets Control may issue licenses permitting or directing such sales or liquidation in appropriate cases.
(f) Funds subject to this section may not be held, invested, or reinvested in a manner that provides immediate financial or economic benefit or access to any person whose property or interests in property are blocked pursuant to §594.201(a), nor may their holder cooperate in or facilitate the pledging or other attempted use as collateral of blocked funds or other assets.

31 C.F.R. § 594.203