"What banks need to be careful about now is ... Not doing business with sanctioned parties in Iran, and there are some that are still sanctioned for terrorism," said Chris Backemeyer, the principal deputy coordinator for sanctions policy at the US Department of State.
Speaking to reporters in Dubai, Backemeyer, who was one of the negotiators in the talks that resulted in Iran's nuclear deal with major powers, named the Revolutionary Guards in particular.
But banks still face restrictions in the United States on handling transactions involving Iran.
"They (banks) need to be careful, and they need to be careful not to route things through the United States because... It's still prohibited for transactions (to go) through the United States," Backemeyer said.
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The US official was in Dubai as part of a delegation to explain the deal with Iran to businesses. He pointed out that the US embargo on Iran "remains in place."
He said there are a few exceptions to the US embargo introduced by the Iran nuclear deal, including the export of aircraft of US origin for explicit civilian use.
The exemptions also allow the importation of Iranian foodstuff and handicraft, and permit subsidiaries of US firms incorporated abroad to engage with Iran as far as the JCPOA permits.