Secretary of State John Kerry testifies before the House Foreign Affairs Committee on Dec. 10 about Iran and sanctions. (T.J. Kirkpatrick/Getty Images)

The Obama administration cracked down Thursday on oil and shipping companies accused of helping Iran evade economic sanctions, a move that came as the White House appeared to gain ground in its fight to keep Congress from enacting even tougher restrictions that threaten to derail nuclear talks with the Islamic republic.

The enforcement actions against Asian, European and Iranian firms were announced moments before two of the administration’s top experts on Iran appeared before a Senate panel to warn again that the imposition of further sanctions could doom any chance of a final agreement on permanent limits to the country’s nuclear program.

The officials cited the new enforcement measures as evidence that the administration intends to keep steady pressure on Iran to achieve that goal.

“We will continue to take action against those who evade, or attempt to evade, our manifold sanctions on Iran,” David S. Cohen, the Treasury Department’s undersecretary for terrorism and financial intelligence, said at a hearing of the Senate Banking Committee. Despite an agreement reached last month in Geneva granting Iran temporary relief from some sanctions, “Iran is still off-
limits” for most oil and banking transactions, Cohen said.

Banking Committee Chairman Tim Johnson (D-S.D.) later appeared to back the administration’s request for a delay in new sanctions, saying the call for a diplomatic pause was “reasonable.”

At a Senate Banking committee hearing Thursday, Undersecretary of State for Political Affairs Wendy Sherman explained steps involved in the p-five-plus-one joint agreement limiting Iran's nuclear program. (Associated Press)

“A new round of U.S. sanctions now could rupture the unity of the international coalition against Iran’s nuclear program,” the senator said. “Existing sanctions will continue to bite, and to bite hard.”

In another apparent victory for the administration, a key House Democrat withdrew support for a proposed bipartisan resolution that would have demanded far deeper concessions from Iran, including a dismantling of all its uranium enrichment facilities. Rep. Steny H. Hoyer (Md.), the Democratic whip, decided that the “timing was not right to move forward this week” on such a resolution, spokeswoman Stephanie Young said.

Hoyer’s withdrawal sharply diminished the chances that the resolution will be put to a vote before the Christmas recess, congressional officials said.

The enforcement measures announced Thursday by the Treasury Department targeted firms alleged to have assisted Iranian companies by transporting petroleum products or acting as middlemen in transferring payments to Iranian banks — actions that violate economic sanctions enacted over the past two years. Administration officials also blacklisted several Iranian companies involved in the country’s uranium-­mining sector.

A senior U.S. official, in briefing reporters about the measures, sought to distinguish the administration’s efforts to enforce existing sanctions from the attempts by some in Congress to impose additional restrictions on Iran’s battered economy.

“The sanctions framework that remains in place will allow us to continue to apply pressure on the Iranian government and the Iranian economy,” said the official, who spoke on the condition of anonymity to describe the legal underpinnings of the enforcement measures. By cracking down on Iran’s foreign business partners, the official added, the White House was sending a “loud and clear message that we are quite serious about taking action against anyone who engages in this activity.”

The announcement of the enforcement actions came amid maneuvering in Congress as lawmakers debated whether to push for new sanctions legislation in defiance of the administration’s warnings. Key Republicans, including Sen. Lindsey O. Graham (S.C.), have criticized the interim nuclear agreement with Iran on grounds that it doesn’t go far enough in dismantling the country’s nuclear infrastructure.

Iran has said that its program is for peaceful energy-producing purposes, but the United States and others suspect it is seeking to develop the capacity to build a nuclear bomb.

The legislative skirmishing coincided with talks in Geneva on how to implement the first phase of the landmark nuclear deal reached Nov. 24. That agreement freezes key parts of Iran’s nuclear program for six months while diplomats try to hammer out a permanent accord on deeper cuts to the program.

Negotiators from Iran and six major powers have been hashing over details such as schedules for intensive inspections of the country’s nuclear facilities, including its uranium enrichment plants and uranium mines. The head of the Iranian delegation to the technical talks in Geneva, Hamid Baeedinejad, described the tone of the meetings as “very smooth.”

Wendy Sherman, the undersecretary of state for political affairs and one of the two administration officials who testified at Thursday’s Senate hearing, said those talks could collapse if Congress imposes new sanctions. Both Iran and key U.S. allies abroad would perceive the United States as acting in bad faith, potentially undermining international support for the very sanctions that brought Iran to the negotiating table, she said.

Although lawmakers can act quickly later if Iran reneges on its promises, Sherman said, it is vital for Congress to “keep its powder dry for the moment so we can keep our partners on board.”

Ed O’Keefe contributed to this report.