The financial obstacles are bad news for Iran’s moderate president, Hassan Rouhani, who ran for office on promises of an economic revival fueled by the nuclear deal and the lifting of sanctions. Hard-liners have pointed to the financial complications as evidence that the United States can never be trusted, and that Mr. Rouhani has been deceived into making unreciprocated concessions.

Indeed, American officials have discounted talk of an Iranian windfall. On Monday, Secretary of State John Kerry told the Jewish lobbying group J Street in Washington that Iran had so far received only $3 billion after the nuclear deal, far less than what some Republican critics of the deal claimed were $150 billion in frozen assets. “We thought it would be $55 billion, but guess what,” Mr. Kerry said, holding up three fingers.

United States officials say — and most European experts agree — that it is not only the sanctions that are blocking Iran’s access to the American financial system but also Iran’s failure during its years of isolation to update laws governing money laundering and prohibitions against the financing of terrorism.

European oil companies, whose capital and expertise Iran desperately needs to revitalize its flagging oil industry, are not making any deals until the Treasury provides clarity over the use of dollars for sales of Iranian oil, diplomats and industry insiders say.

The European Union’s foreign policy chief, Federica Mogherini, flew to Tehran last week to try to convince Iranian officials that the bloc is doing all it can to “reassure” major European financial institutions that it is safe to work with Iran.

But European banks will still have to contend with American regulatory requirements. According to a financial disclosure law, all banks worldwide have to disclose whether their customers are American citizens, to prevent tax evasion. If they fail to do so the banks’ assets can be seized.

Several million people hold dual Iranian and American citizenship. Because the Iranian government is reluctant to allow information about its citizens to be shared with the United States, experts say, it is unlikely that it will comply with the act — another complication for European banks.