More than two months after international nuclear sanctions on Iran were supposed to have ended, frustration is deepening that few trade deals are going through as foreign banks shy away from processing transactions with the country.

Iranian hopes of rapidly ending the country's economic isolation are fading as particularly European banks - some of which have already been hit by hit huge US fines for sanctions busting - fear falling foul of the many other restrictions imposed by Washington that remain in force.

Supreme Leader Ayatollah Ali Khamenei has accused the US of foot-dragging following the official implementation in January of a nuclear deal with major powers.

"The Americans have not acted on their promises and (only) removed the sanctions on paper," he said in a televised address on Sunday, complaining that international financial transactions faced problems because banks "fear the Americans".

Many nuclear-related sanctions did end when the 2015 nuclear deal was implemented on January 17, including measures imposed by the European Union and rules allowing US authorities to go after foreign companies and individuals dealing with Iran.

Agreements on a number of major contracts have been announced with great fanfare, with Tehran hoping relief from the crippling sanctions will lead to billions of dollars in trade and investment, reviving the economy and raising Iranians' living standards. However, significant sums have yet to start flowing.

US banks are still forbidden to do business with Iran and while lenders based elsewhere are not covered by this ban, major problems remain. Chief among these are rules prohibiting transactions in dollars - the world's main business currency - from being processed through the US financial system.

The Iranian business community believes the US has failed to spell out exactly what is permitted and what is not, leading to the uncertainty that makes international banks reluctant to process Iranian-linked transactions.

"We have to try to put pressure on America to make this issue clear. Otherwise, removing the sanctions does not mean anything," said Ferial Mostofi, chairwoman of privately-owned Iranian project management firm KDD Group.

KDD Group, which is active in sectors including iron, steel and mining, has noticed greater business interest from abroad, she said, but so far no deals have been concluded.

"If the banking situation stays as today, definitely we shall be facing problems for the payments," said Mostofi, who also chairs the Investment Commission at the Iran Chamber of Commerce.

The US Treasury, which is responsible for enforcing sanctions on Iran, gave no immediate response to a Reuters request for comment.

Iranians based in Dubai, historically one of Iran's main trading partners, complain they cannot get letters of credit to finance deals with their home country, while others have even had their company bank accounts closed in recent weeks.

The problems are also complicating Iran's plans to sell more oil, as well as recover up to $100 billion in assets that had been frozen by the sanctions in foreign bank accounts.

AGREEMENTS, FEW DEALS

Since January, Iran has struck agreements worth an estimated $50 billion with countries including Italy, Japan, South Korea, Russia, Germany and others involving trade, project finance and other investment.

Agreements include a contract to buy 118 Airbus jets worth $27 billion. However, the funding needed to turn agreements into firm deals is another matter.

One Airbus executive told a conference in Paris last month that "we only see the back of banks at the moment", telling them: "Don't be afraid!"

Banks remain deterred by a $9 billion US fine on BNP Paribas in 2014 for violating US financial sanctions and other penalties, and the head of the French banking federation told the conference that lenders had yet to be assured of "complete legal security and clarity".

That will be tough as long as Washington keeps the ban on processing dollar transactions for Iran in the U.S. system.

"Until US sanctions are lifted European banks with major operations in the States, of which there are many, will still be exposed to onerous trade restrictions unless they can prove complete separation of European and US divisions of their business," said George Booth, a partner at law firm Pinsent Masons.

"That's easier said than done. It should not be underestimated the level of internal restructuring required to satisfy this criteria," said Booth, who advises firms hoping to do business with Iran.

Seyed Arash Shahr Aeini, deputy chief executive of the Export Guarantee Fund of Iran, an Iranian government agency, said so far only smaller banks were willing to become involved, and transactions were limited to around $50 million.

"Some small amounts have gone through but the huge amounts will require the involvement of big foreign banks which were active in Iran projects before the sanctions were imposed. They are still reluctant to start doing business with Iran."

In recent weeks SWIFT, the global payments network, has reconnected several Iranian banks to its system, allowing them to resume cross-border transactions with foreign banks four years after they were cut off.

While an important step towards re-integrating Iran into the global financial system, the outcome has appeared patchy so far.

Two banking sources said most international banks were still refusing to accept cheques from holders of accounts at one major Iranian commercial bank that has been reconnected.

Ali Sanginian, chief executive of Amin Investment Bank, Iran's largest investment bank, blamed the delay in reintegrating Iranian lenders on the remaining sanctions, the banks' anxieties and outdated technology used in Iran.

An international banker in the region said his bank's aversion to Iranian transactions had not changed. "Around 85 percent of trade is in US dollars and if you're dealing in dollars you cannot risk that by involvement with Iran," the banker said.

Of the transactions that are happening, some are in euros and other currencies, permitted under the current arrangement.

"Notwithstanding the relaxation of the position, it is still unclear as to whether if you are moving US dollars around they may get held up in the banking system," said James Kidwell, chief executive of shipping group Braemar. "Some people are probably choosing to transact in euros to avoid that problem."

Iran has managed to sell oil to India and other buyers in euros. It told trading partners which owe it billions of dollars that it wants to be paid in euros, Reuters reported last month, citing a source at state-owned National Iranian Oil Co. - Reuters