The Obama administration in mid-January eased several restrictions on doing business with Iran, including former "secondary" sanctions that had threatened to penalize companies outside the US for their business with Iran, as well as some restrictions on Americans seeking to make inroads in the oil-rich country.
Nevertheless, most "primary" sanctions tied to accusations that Tehran supports terrorism remain in effect, blocking US businesses from joining a rush by non-US companies to cash in on Iran's potential revival.
It means that US banks have little access to the oil-rich country compared to their rivals in other countries.
They mostly cannot handle transactions for US and other companies involved in Iran, and the Iranian government and private entities cannot open accounts with US banks.
"Broadly, the US primary embargo on Iran is still in place," John Smith, acting director of the Treasury Department's Office of Foreign Assets Control (OFAC), told a congressional panel on Thursday.
The sanctions still in place were imposed not over Iran's nuclear program but its record on terrorism and human rights.
That means the easing arising from the nuclear deal so far "does not have any impact on us," said an official with one large New York bank who requested anonymity.
"We're still very prohibited from engaging in just about any business activity with Iran except on very limited exceptions."
Several leading US banks, including Bank of America, JPMorgan Chase, Goldman Sachs and Morgan Stanley, declined comment on the issue.
But many remain keen to exploit the Iran opportunity. They have turned to teams of lawyers and other specialists as they plumb the shifting legal terrain.
"We continue to monitor the developments in Iran," said Citigroup spokesman Kamran Mumtaz.
Non-US banks also restricted
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Foreign banks operating in the US too remain hemmed in by the sanctions still in place, because they are forbidden from clearing US dollar-denominated transactions involving Iran through US banks, according to OFAC.
In addition, some 200 Iran-related individuals and entities on a list of "blocked" persons, including large government entities involved deeply in the economy like the powerful Revolutionary Guards.
Non-US companies who provide support to still-blacklisted Iranian entities "may face being cut off from the US financial system," said OFAC's Smith.
In 2014, US officials fined BNP Paribas a record of almost $9 billion for moving payments involving Iranian entities through the US economy.
To avoid such punishments, non-US banks that work in both Iran and the US must isolate Iranian business from their US assets and implement rigid internal controls, law firm Clifford Chance said in a memo advising US banking giant JPMorgan Chase.
"Many organizations looking to trade with, or make investments in Iran, now that sanctions have been eased may experience a tension between business development personnel who may wish to take advantage of the opportunities offered, and those whose responsibilities are for risk management and compliance," said the memo.
US policy has freed banks to make loans in some specific businesses and activities involving Iran, such as to companies that have obtained specific US Treasury permission, or "licenses," to sell airplane parts or to upgrade Iranian civilian aviation engines.
Boeing and General Electric have obtained such licenses.
US banks can also provide financing to US companies that import Iranian foods or carpets, said a New York banker.
But "there are still a number of risks to consider when conducting business under the terms of a specific license," said Howard Mendelsohn, a managing director at Camstoll Group, a consultancy.
US banks must carefully vet all parties involved in a transaction to make sure none are on the Treasury blacklist.
Adding to the caution is the call by Republican presidential candidates to undo the nuclear agreement promoted by President Obama.
If Obama's policy is overturned, the broader sanctions on Tehran could return, jeopardizing US businesses that had pushed in to the county.
"Any mistake that might be made would be subject to a higher level of criticism," said a sanctions expert who has consulted with big Wall Street banks.