The United States is not opposed to foreign banks doing business with Iran in line with the terms of last year's historic nuclear deal between Tehran and world powers, US Secretary of State John Kerry said on Friday.

"The United States is not standing in the way, and will not stand in the way, of business that is permitted in Iran since the (nuclear deal) took effect," Kerry told reporters before meeting with Iranian Foreign Minister Mohammad Javad Zarif.

Kerry said he was trying to clear up uncertainty in the business community outside the United States about investing in Iran. The Iranian government has complained about not getting the full economic fruits of the July 14, 2015 nuclear deal.

"There are now opportunities for foreign banks to do business with Iran," Kerry said in New York, where he attended a UN signing ceremony for the Paris climate accord. "Unfortunately there seems to be some confusion among some foreign banks and we want to try and clarify that."

"Among the nuclear-related sanctions that were lifted were those that prevented Iran from engaging with non-US banks, including getting access to Iran's restricted funds," Kerry said.

The only exceptions, he added, would be banks and companies blacklisted by US authorities. He said it was understandable that some companies might need time to feel confident about doing business in Iran, noting that Tehran also needed to take steps to modernize its banking system.

The nuclear agreement between Iran and six world powers allowed for the easing of some sanctions imposed by the United States, European Union and United Nations in return for Tehran curbing its nuclear program.

Tehran has called on the United States to do more to remove obstacles to the banking sector so that businesses feel comfortable with investing in Iran without penalties.

Western firms also are wary of doing business in Iran because of the prospect that seemingly innocent local companies could have links to banned entities controlled by the increasingly powerful Islamic Revolutionary Guard Corps (IRGC), economists and officials say.

Zarif said he hoped Kerry's clarification would help, though he noted that "we will continue to have differences with the United States."

"We hope that with this statement by Secretary Kerry .... now we will see serious implementation of all (nuclear deal) benefits that Iran should (enjoy) from this agreement," Zarif said.

He added that Tehran hoped Kerry's words would "open the difficult path that has been closed because of concerns that banks have about the US approach toward implementation of commitments" under the nuclear deal.

Kerry said there remained some "serious differences" with Iran on implementing the deal. "Those have to be the subject of future discussion, but its important for people to understand that an agreement is an agreement," he said.

Zarif and Kerry met earlier this week at the U.N. to discuss sanctions relief.

Current US policy bars foreign banks from clearing dollar-based transactions with Iran through US banks. But US officials have said the Obama administration is considering ways in which non-US companies could use the dollar in some business transactions with Iran.-Reuters