By DAVID E. SANGER and MICHAEL R. GORDON Published: December 12, 2013 WASHINGTON — Under pressure from Congress to demonstrate that it is not easing up on sanctions on Iran’s oil sector or on its nuclear and missile programs, the Obama administration on Thursday announced an expanded list of companies and individuals that it said it would target to block their trading activities around the world. Among the newly penalized companies is a Singapore-based firm called Mid Oil Asia, which is accused of helping the National Iranian Tanker Company make payments for services through money transfers that made no mention of the vessels that were aided, or their Iranian ownership. Another Singapore company, Singa Tankers, is accused of helping Iran make “urgent payments.” The location of both companies is notable because Singapore often prides itself on running a carefully regulated shipping and banking system.
Five companies are accused of helping Iran’s nuclear and missile program, including an Iranian firm, the Eyvaz Technic Manufacturing Company, that the United States said had procured some of the most sensitive and hard-to-build components for Iran’s nuclear centrifuges. The centrifuges are the machines that, spinning at supersonic speeds, enrich uranium; over the years the United States has sought to undermine the effort with sanctions, faulty parts and cyberattacks.
Another firm is accused of helping Iran obtain components for its heavy-water reactor facility, which officials fear will ultimately give Iran another pathway to a bomb capability, using plutonium.
The administration’s announcement of its enforcement actions appeared to be timed to set the stage for a Senate Banking Committee hearing on the Iran nuclear talks and the United States sanctions policy on Thursday morning.
Wendy R. Sherman, the senior State Department official who led the American delegation at the nuclear talks with Iran, and David S. Cohen, the senior Treasury Department official who oversees the enforcement of sanctions on Iran, testified to the panel.
The aim of the interim agreement that was reached last month in Geneva is to freeze much of Iran’s nuclear program for six months so that international negotiators can pursue a more comprehensive accord.
That interim agreement, however, has not yet formally gone into effect. Ms. Sherman said that the precise start date was being taken up in technical talks, but that the agreement should start to take effect in the next several weeks.
The interim agreement can also be extended for an additional six months by mutual consent if negotiators need more time to pursue a follow-on agreement.
There has been concern in Congress that the talks may be dragged out and that sanctions may erode as negotiators seek such an agreement.
Mr. Cohen cited the enforcement actions announced on Thursday as an indication that the Obama administration would not let its guard down and would maintain core banking and oil sanctions as it pursues a more comprehensive accord.
“We are watching closely, and we are prepared to take action against anyone anywhere who attempts to violate our sanctions,” Mr. Cohen said.
Senator Tim Johnson, the South Dakota Democrat who is the chairman of the Banking Committee, reaffirmed that he accepted the administration’s argument that it would be counterproductive to legislate new sanctions at this time because it would alienate the United States’ negotiating partners and poison the atmosphere for future talks.
Senator Bob Corker, Republican of Tennessee, criticized the interim deal because it indicates that Iran would be permitted to continue enriching uranium in a follow-on accord, albeit under strict limits.
Mr. Corker has introduced legislation that would give the White House no more than 180 additional days to conclude the comprehensive agreement. If a deal was not reached in that time frame, any sanctions that had been relaxed would be reimposed.
But Mr. Corker hinted that he did not expect much support for new sanctions legislation in the Banking Committee. “We are participating in a bit of a rope-a-dope today,” he said.
On Tuesday, Secretary of State John Kerry came under sharp criticism from Democrats and Republicans on Capitol Hill, who are threatening new sanctions against Iran that the administration fears will undermine the preliminary agreement it reached to freeze major elements of the country’s nuclear program.
In return, President Obama agreed to a temporary lifting of sanctions on auto production and petrochemical sales, but not on Iran’s oil sector, its greatest source of revenue.
Critics of the preliminary deal, however, have said that reversing course on even a limited set of sanctions will undermine the pressure on Tehran, and make less likely a final deal to roll back the nuclear program.
Mr. Obama took the other side of the argument on Saturday, telling an audience at an Israeli-American forum at the Brookings Institution that unless the Iranian people saw the prospect of some economic relief, there would be no political constituency inside Iran for giving up the key elements of a potential nuclear weapons capability.
But the administration has also been eager to show that it is continuing to apply pressure on the nuclear program, on banks violating sanctions on financial transactions, and on front companies that are helping Iran evade oil sanctions.
“There is no way Iran is open for business,” a senior administration official told reporters in a conference call on Thursday morning, announcing the new targets of sanctions. He added that when it came to the core sanctions, “we haven’t let up, and we aren’t letting up.”
A version of this article appears in print on December 13, 2013, on page A4 of the New York edition with the headline: White House Punishes More Firms Over Iran Sanctions. ------ ...