HELSINKI, May 31 (Reuters) - The United States must do more to assure banks that they can do business with Iran, Iranian Foreign Minister Mohammad Javad Zarif said on Tuesday.

Iran is struggling to access financing from abroad as many large banks fear breaking the remaining U.S. restrictions, which prohibit trade with Iran in dollars and bar Iranian access to New York's financial system.

"It seems that there is a psychological barrier," Zarif told reporters during his visit in Helsinki.

"Some European countries, even European banks, continue to be concerned about retribution by the United States. I believe that (in) the United States, they need to go further in order to provide reassurances to the banks that this will not take place."

International measures against Iran were lifted in January as part of the deal with world powers under which Tehran curbed its nuclear programme. But the banking industry has been left cautious over fines incurred for sanctions breaches in recent years.

(Reporting by Tuomas Forsell; writing by Jussi Rosendahl Editing by Jeremy Gaunt) ((jussi.rosendahl@thomsonreuters.com; +358 9 6805 0248; Reuters Messaging: jussi.rosendahl.thomsonreuters.com@reuters.net))