Washington on Thursday blacklisted a dozen overseas companies and individuals for evading sanctions on Iran, as US lawmakers were persuaded to hold off on new punitive measures against the country.

Only hours after the surprise blacklist announcement, seen as a clear warning to Tehran to comply with a nuclear deal, two top senators bowed to White House pleas not to introduce new sanctions.

The pair acknowledged more sanctions could "rupture" unity among global powers seeking to rein in Iran's suspect nuclear enrichment program.

"Let me be clear. I support strong sanctions, and authored many of the US sanctions currently in place," said Senate banking committee chairman Tim Johnson, who has been in the spotlight over whether he would introduce a new sanctions regime this year, as several senators have sought.

"But I agree that the administration's request for a diplomatic pause is reasonable," Johnson told a hearing.

Under Thursday's designations, US citizens or companies are barred from engaging in transactions with any of the targeted firms and individuals whose assets will be frozen in the United States.

"Today's actions should be a stark reminder to businesses, banks and brokers everywhere that we will continue relentlessly to enforce our sanctions, even as we explore the possibility of a long-term, comprehensive resolution of our concerns with Iran's nuclear program," said David Cohen, Treasury under secretary for terrorism and financial intelligence.

Those listed include the Singapore-based Mid Oil Asia and Singa Tankers, both companies accused of helping Iran transfer badly-needed funds to a foreign bank on behalf of the National Iranian Tanker Company.

Ukrainian national Vitaly Sokolenko and his Odessa-based firm Ferland Company Limited were also cited for helping to broker the sales of Iranian oil and transfer the crude ship-to-ship.

Secretary of State John Kerry this week returned twice to Capitol Hill seeking to dissuade lawmakers from drawing up new legislation and urging that diplomacy be given time to work.

Under an interim deal reached in Geneva last month, Iran agreed to freeze parts of its suspect nuclear program in return for some $7 billion in sanctions relief as it negotiates a final, comprehensive accord to dismantle its nuclear weapons capability.

The United States also agreed to refrain from slapping new sanctions on Iran, but senior administration officials argued that Thursday's measures were taken as part of the existing sanctions regime which had forced Tehran to the negotiating table.

"Sanctions pressure will be essential as we seek to negotiate a comprehensive long-term resolution," a senior administration official told reporters on a conference call.

"The message to the Iranian government remains clear, the only escape from increasing sanctions pressure, the only path out of deepening economic distress is to negotiate."

Despite Johnson's move Thursday he made clear that he was already crafting new sanctions behind closed doors which could be acted on quickly if Iran fails to comply with the terms of the November 24 deal.

Johnson's fellow Senate Democrat Robert Menendez and Republican Mark Kirk have also been drafting a bill that they want to see passed sooner rather than later imposing tough new economic sanctions if Iran reneges on the six-month accord.

But officials said the timing of Thursday's designations were coincidental and not aimed at sending a message to Congress to back off new sanctions.

The US State Department also named five Iranian entities and several people that it alleges provide goods and services that contribute or risked contributing to Iran's nuclear program or its development of ballistic missiles.

State Department deputy spokeswoman Marie Harf welcomed Johnson's decision as "in the best interest of our diplomacy and our diplomatic efforts."

She also revealed that US officials who were in Vienna to hammer out the technical details of the November deal, as well as a start date, had given their Iranian counterparts a heads-up that the designations were coming, in an unusual move.

There were warnings too that the limited sanctions relief promised under the Geneva deal did not mean that Iran was now open for international business again.

"There's no reason to think that there's great business opportunity in Iran today. The majority of their banks remain isolated from the international financial system," the senior administration official, who asked to remain anonymous, said on the conference call.

And he warned that the energy sector -- most notably Iran's vast oil reserves -- remained subject to stringent sanctions.