After the U.S. walked out of the Iran deal, the U.S. will be targeting Iran's crude oil sales, and sanctions previously lifted under the deal will be re-imposed following a 180-day wind-down period, the U.S. Treasury said.

European buyers are not in an immediate rush to replace Iranian supplies due to that wind-down period, with sanctions expected to kick in in November. All buyers report that they are complying and will comply with any sanctions imposed on Iranian trade, and some of them expect that banking issues will arise from the sanctions, such as the availability of trade finance.

Marta Llorente, a spokeswoman for Spanish oil company Cepsa, one of Iran's customers in Europe, told Reuters:

"At this moment, our trading activity is business as usual."

Italy's Eni also continues to buy Iranian oil and it is buying 2 million barrels of oil per month from Iran under a deal that expires at the end of the year.