WASHINGTON: The United States will try to help foreign companies become more at ease doing business with Iran after last year's nuclear deal, but Tehran must also reassure them it is a safe place to invest, President Barack Obama said on Friday.

Iranian officials have complained the country is not getting the full economic fruits of the landmark July 14 deal, which offered sanctions relief in return for Iran's curbing its nuclear programme.

Speaking to reporters at the close of a two-day summit on nuclear security, Obama said foreign companies needed to get used to the idea of working with Iran after many years in which they faced high penalties if they did so and violated sanctions.

He also said US Treasury Secretary Jack Lew, along with his counterparts from the five other world powers that negotiated the deal with Iran, would provide "clarity to businesses about what transactions are, in fact, allowed."

Obama said it would take several months for companies to feel confident they can deal with Iran.

However, he said the United States was not looking to permit the use of the US financial system for dollar-denominated transactions with Iran, and said foreign companies could work through European banks.

Most US sanctions against Iran remain in place, severely limiting what business US companies can do with the Islamic Republic and placing the US financial system essentially off limits for transactions with Iran by non-US companies.

"Some of the concerns that Iran has expressed, we are going to work with them to address," Obama said, adding that it was "not necessary that we take the approach of them going through dollar-denominated transactions."

He also placed the onus on Iran to do more to reassure the world that it was a safe place to invest.

"Iran so far has followed the letter of the agreement, but the spirit of the agreement involves Iran also sending signals to the world community and businesses that it is not going to be engaging in a range of provocative actions that might scare business off," he said.

"When they launch ballistic missiles with slogans calling for the destruction of Israel, that makes businesses nervous," he said.

Iran in early March test-fired ballistic missiles, and the US ambassador to the United Nations, Samantha Power, has said that Iranian military leaders have claimed the missiles "are designed to be a direct threat to Israel."

Iran's Supreme Leader Ayatollah Ali Khamenei on March 10 said the Iranian economy had not yet benefited from the Western trade delegations visiting Iran, saying: "We are expecting to see some real improvements. Promises on paper have no value."

One underlying reason for the Iranian frustration is the fact that non-US companies that are now allowed to trade with Iran find they cannot do so without touching the US financial system, which remains off limits.

As a result, US officials have said the United States is considering easing some sanctions to permit non-American companies to have some access to the US financial system for dollar transactions involving Iran.

US House of Representatives Speaker Paul Ryan, a Republican, on Friday urged Obama to reject any such moves.

"These reports are deeply concerning, to say the least," he said in a statement. "As Iran continues to undermine the spirit of its nuclear agreement with illicit ballistic missile tests, the Obama administration is going out of its way to help Tehran reopen for business. The president should abandon this idea."