By Mark Gregory
International business reporter, BBC World Service
The United States is waging an undeclared financial war on Iran as part of efforts to persuade the Tehran government to abandon alleged plans to acquire nuclear weapons.
It is being run from an ornate, grey building located close to the White House in Washington - the headquarters of America's finance ministry, the Treasury Department.
"What we're trying to do is make it difficult for Iran to use the global financial system to pursue illicit conduct," explains Stuart Levey.
As undersecretary for terrorism and financial intelligence at the Treasury, he is in charge of a US campaign that has two main prongs.
The first is formal financial sanctions on Iran, as agreed at the United Nations. These are limited in scope - the sort of thing that even Iran's friends, China and Russia, are willing to accept.
The UN sanctions prohibit transactions with a small number of Iranian banks, companies and individuals said to be directly involved in the country's nuclear programme or in support for terrorism.
But there is also a second, potentially more powerful, element. Since September 2006, US officials have been travelling the world talking to banks and company bosses. They aim to persuade business to voluntarily abandon or scale back all dealings with Iran.
Mr Levey, who is spearheading the Treasury's campaign, insists he is already getting results.
"There is significant evidence that it's working in the sense that Iranian business is being subjected to greater scrutiny and it's more difficult for them to operate," he says.
"A number of major financial institutions have cut off doing business with certain Iranian banks or with Iran entirely."
American financial pressure shows up in small ways. For example, international banks have become reluctant to issue letters of credit on Iranian trade, or only on exorbitant terms.
Letters of credit guarantee payment in international commerce. The alternative is paying in cash.
Consequently, Iranian companies often find themselves unable to carry out transactions in dollars, still the most important currency for trade.
The aim is to squeeze the Iranian economy so that the nation's leaders will decide the price of developing nuclear weapons is just too high - although Iran, of course, denies that it is trying to acquire a nuclear arsenal in the first place.
So is American financial pressure as effective a tool as US officials claim?
Dubai is a good place to find out - a rich Gulf city that is Iran's economic gateway to the world.
A quarter of Dubai's population are Iranian and much of Iran's trade goes through Dubai. The relationship is similar to China's link with Hong Kong.
Iranian businesses in Dubai are universally reluctant to talk about such a sensitive topic. They risk offending either the Americans or the government in Tehran, more or less whatever they say.
Much Iranian economic activity is channelled through Dubai
It took several days of intensive effort to find a few brave souls willing to speak openly.
Barmak Besharaty is an Iranian with an American passport. His business is to arrange huge loans for property deals and he works closely with international banks.
For customers from Dubai, Pakistan, Britain, or any of 70 other nationalities operating in this Middle East entrepot, there is no problem. But when it comes to persuading banks to stump up loans for Iranian customers, he says it is a different story.
"I won't say Iranians can't raise money at all, but the difficulty level has been raised substantially," he observes.
Mr Besharaty cites a specific example, claiming that the Dubai office of HSBC, the world's fourth biggest bank, is refusing to give loans to Iranians.
"They do more business in the United States than in Iran," he says.
"And the United States says, 'If you want to operate here without difficulty, it would be more prudent for you to not do business in Iran.' And I think that HSBC listens to that."
Interestingly, when this was put to HSBC they did not exactly deny it.
The bank's head office in London issued a statement: "At the present time, all financial institutions do need to be careful about the business they do in Iran.
"There are sanctions in place from several bodies, notably the United Nations, and any bank that is, as we are, committed to abiding by all of the regulations that apply to us has to be cautious."
Nasser Hashempore is another Iranian prominent in business in Dubai. He is a senior figure in the Iranian Business council, a lobby group.
Mr Hashempore says Iranian businesses are all worried about US financial pressure, but so far, they are finding ways round it.
Mr Hashempore confirms that many Iranian firms are barred from doing deals in dollars, but they cope by switching to euros or yen instead.
He also says that many Iranian companies are able to pass themselves off as local Dubai businesses, which means they can get loans and letters of credit from international banks.
Under Dubai law, foreign enterprises are required to have a local business partner, nominally with a 51% stake in the firm.
These partners are usually paid a fee and play no active role in running the business, but for legal purposes, the firm can say it is locally-owned and thus avoid American financial pressure.
Faranak Mahmoudi is a rare example of an Iranian woman with a senior role in business. She has lived in Dubai since 1990 and specialises in advertising and promotions.
Her clients include well-known European consumer products companies who want to tap into the Iranian market.
Faranak Mahmoudi says foreigners are flocking to invest in Iran
"Actually, nobody is taking 'American pressure' seriously yet," she says. "In fact, I see more investment from foreign companies in Iran. I've never had so many requests for media campaigns in Iran as I've had during the last two months."
Iranian firms in Dubai are putting up a brave face, but it is clear that they fear America's financial squeeze could get a lot worse.
The key to Iran's prosperity though is its oil and gas sector. Iran is an oil economy. It has the world's second-largest confirmed oil reserves and huge quantities of untapped natural gas.
But it desperately needs energy investment.
The obvious way to strangle the economy is to hit the vulnerable oil and gas industries. And yet, for international energy companies, Iran is potentially a mouth-watering prize.
European energy giants - including Shell, Spain's Repsol and Total of France - are eyeing up big up Iranian deals, as are Chinese and Malaysian oil firms. But in so doing, they all risk alienating the US, the world's only super-power.
Most oil companies do business in the US - and that would be under threat if they get too close to Iran.
Hence, oil firms are in a difficult situation, observes David Kirsch, a former official at the US State Department who now works for a Washington consultancy, PFC Energy.
"You've already seen some local calls in the US for protests against oil companies, Shell in particular, for their investments in Iran," he says.
"These could become far more widespread, particularly if politicians in Washington seize upon these investments as a rallying call."
But on the other hand, Iran's huge energy reserves are hugely significant for the oil industry.
Iran has started enriching uranium gas, nuclear watchdogs say
Mr Kirsch says many oil firms would take the risk of upsetting the US if others were doing the same.
"What you're seeing is a strange sort of dance with some of these energy companies and they're all hoping that another company will be the first one in to become the lightning rod for the US reaction," he explains.
"The first company that does break ranks and makes a major investment will lead to an opening of the floodgates," he says, meaning that if one oil company does a deal with Iran, lots of others may follow.
So what does all this add up to? A Treasury campaign of squeezing business with Iran; Iranian businesses in Dubai bending but not breaking under the strain; International energy firms calculating the odds on whether to defy the US. And all this underpinned by small scale UN sanctions.
Will this be enough to make Iran to abandon its alleged nuclear plans? The answer is probably no - at least if Tehran is determined enough to put up with some economic pain.
How much pain depends on the US commitment. After all, the US could decide to punch with the full force of its $12 trillion economy - and that would make life exceedingly difficult for the government in Tehran.