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A senior US official dismissed reports Tuesday that Washington plans to allow Iran access to the US financial system or to buy the latest Russian warplanes.

Responding to skeptical US lawmakers, Undersecretary of State Thomas Shannon insisted that the United States would maintain sanctions pressure on the Islamic Republic.

But he confirmed that the State Department is attempting to "clarify" the terms of its financial sanctions to foreign banks now looking to do business with Iran.

"The rumors and news that has appeared in the press that the US is preparing to reinstate Iranian access to the US financial system are not true," he said.

In January, the United States and the European Union implemented their side of last year's Iran nuclear deal, lifting a raft of sanctions.

Under the accord, Iran had divested itself of its nuclear fuel stockpile, dismantled much of its enrichment capacity and put a bomb beyond its immediate reach.

In return, the West dropped many sanctions, giving Iran access to tens of millions of dollars in frozen assets and allowing it to double its oil exports.

But Washington maintains other sanctions designed to punish Tehran for maintaining a banned ballistic missile program and for arming groups designated by the US as "terrorist organisations".

Now Tehran is chafing under these ongoing sanctions, with hardliners accusing President Hassan Rouhani of having sold his country out for little reward.

We have found that as Iran seeks those funds, there are banks unclear of the nature of the regulatory structures. - Undersecretary of State Thomas Shannon

Since the lifting of nuclear sanctions, European and other international banks only gingerly returned to the Iranian market, fearing their American operations will fall afoul of US prosecutors.

Secretary of State John Kerry and to a lesser degree President Barack Obama have hinted at sympathy for the complaints, insisting the United States will keep its side of the deal.

But this has led to concerns in Congress - and conflicting reports in Washington - that the administration is preparing new concessions to Iran.

As the Senate Foreign Relations Committee convened on Tuesday, Senator Bob Corker asked Shannon to explain these "rumors out of the administration."

Shannon insisted that "as far as I know" there are no plans to allow Iran to use the US financial sector for transactions in dollars.

But he confirmed that the administration planned to reassure foreign banks seeking to do business with Iran about where they stand in relation to US law.

"As Iran attempts to access money being made available to it through the lifting of sanctions, there will be instances in which we have had to help Iran access that money by clarifying regulations under which money can be transferred to them," he said.

"We have found that as Iran seeks those funds, there are banks unclear of the nature of the regulatory structures."

Corker told Shannon "there was some concern there was a little bit of a wink and nod going on where we were saying to institutions that we are not coming after you."

He also warned that he suspected some US officials "have developed relationships and are trying to bend this in a way that will benefit Iran."

This was a dig at Kerry, who has a channel of communication to his Iranian counterpart, Foreign Minister Mohammad Javad Zarif.

Shannon insisted Washington will continue to impose tough sanctions on Iran's ballistic missile program and interference in the Middle East.

But he added that both Kerry and Treasury Secretary Jacob Lew have "been clear about the importance of ensuring Iran has access to the assets now open to them."

He also warned that Washington would veto any Russia plan - as reported in Iranian media - to sell Tehran advanced Sukhoi Su-30 jet fighters.

"The sale of this type of aircraft is prohibited without the approval of the UN Security Council and we would not approve it," Shannon said.