The United States is not standing in the way of foreign banks doing business with Iran, a senior U.S. official said, but his comments appeared unlikely to satisfy frustrated businessmen and Iranian officials.

Most international sanctions against Iran's economy were lifted in January after Tehran implemented a deal with world powers to curb its nuclear program. But Washington kept some sanctions that were originally imposed over missile proliferation and alleged support of terrorism.

The fear of being caught up in those remaining sanctions has deterred most foreign banks from restoring links with Iran, angering the Iranian government, which complains it is not getting economic benefits it was promised in the nuclear deal.

Chris Backemeyer, Principal Deputy Coordinator for Sanctions Policy at the U.S. Department of State, said U.S. officials were meeting business leaders around the world to assure them that Washington was complying with the nuclear agreement.

"We've tried to make it 100 percent clear," Backemeyer told reporters in Dubai, where he was meeting local and international companies to explain what the U.S. saw as legitimate business with Iran and what it viewed as illegitimate.

He said that while U.S. banks were still banned from dealing with Iran as part of a trade embargo that remains in place - effectively blocking U.S. dollar transactions, since they would ultimately be cleared in the United States - Washington would not penalize foreign banks for doing business in other currencies.

However, Backemeyer also said foreign banks would need to demonstrate they had performed due diligence to ensure they were not doing business with sanctioned entities in Iran, such as companies linked to the Revolutionary Guards.

That appeared unlikely to reassure bankers from Europe, the Middle East and other countries, who say the remaining sanctions are so complex, and the risks of errors in due diligence so high, that they still don't dare to engage with Iran.

"It's all very well for the U.S. government to say this, but the reality is that it's still dangerous ground to do business with Iran as an international bank," said a foreign banker in Dubai when asked about Backemeyer's comments.

"The large international banks have a low risk appetite because of the legacy of fines and deferred prosecution arrangements against them, and it would take a lot for them to change their position."

The banker was referring to billions of dollars of fines paid by international banks to U.S. authorities in the past several years to settle charges that they failed to prevent money laundering, tax evasion and other abuses.

"GOOD FAITH PARTNER"

Iran's supreme leader, Ayatollah Ali Khamenei, on Sunday accused the United States of undermining the nuclear agreement by deliberately sowing uncertainty among foreign banks. This was making it hard for Iran to obtain billions of dollars of its funds that were frozen by the sanctions, he charged.

"In Western countries and places which are under U.S. influence, our banking transactions and the repatriation of our funds from their banks face problems ... because (banks) fear the Americans," he said.

Backemeyer denied that. "We are a good faith partner," he said. "It is not in our interest just to sanction for no reason, so we're not going to be in a position where we're looking to trick someone, that's never been our objective."

Nonetheless, he said Washington could not tell individual companies how much due diligence they should do to insulate themselves from remaining U.S. sanctions, because that would depend on the nature of their business.

"Banks, because they have to be used for any sort of financial transaction, by definition have the potential to get exposed to all sorts of risks," he said.

Asked about companies' concern that the nuclear sanctions could "snap back", Backemeyer said the United States would not reimpose them as long as Iran continued to adhere to its commitments.

"Snap-back is a mechanism that we negotiated in order to deter Iranian non-compliance, not one to give us some secret way of re-imposing sanctions on Iran because we felt like it."