Many foreign banks are cautious about resuming trade with Iran following January's nuclear deal because they fear being caught up in ongoing US sanctions, Reuters reported.

Rebuilding the confidence of the banks that the United States will not re-intervene in their relations with Iran may require some further assurance from the United States, Iranian Foreign Minister Mohammad Javad Zarif said at Chatham House in London.

"We don't need any more legalese - we need clear precise assurances that banks can do business with Iran," he said. "I hope that is fast coming because if not it would be a problem of implementation."

Washington still prevents US nationals, banks and insurers from trading with Iran and also prohibits any trades with Iran in US dollars from being processed via the US financial system.

European banks are also cautious - with some, including Deutsche Bank, remembering past fines from US regulators for breaking sanctions.

Resuming UK trade

When asked about trade with Britain, Zarif said that good economic and commercial relations could resume.

"Iran and Britain have had traditionally good commercial and economic relations and I think those can resume," Zarif said. "We need to work together on moving the political relations forward."

Iran has 9.3 percent of the world's proven oil reserves, the fourth largest after Venezuela, Saudi Arabia and Canada, and 18.2 percent of the world's natural gas reserves, bigger even than Russia's 17.4 percent share, according to the BP Statistical Review of World Energy.