The US announced Thursday that it was penalising more than a dozen companies and individuals for conducting prohibited business with Iran, barely an hour before it implored Congress yet again not to enact any new sanctions on Tehran.

More than a dozen entities, from Ukraine to the Philippines, are now under economic penalties for dealing with Iran’s oil exports and illicit weapons programs. The Treasury Department’s sanctions chief, under-secretary David Cohen, said the new penalties show that economic pressure would continue on Iran even as negotiators spend the next six months attempting to finalise an accord to permanently stop Tehran from developing a nuclear weapon.

“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,” Cohen said.

The announcement came barely an hour before Cohen and his State Department colleague Wendy Sherman, one of the negotiators of the first-step deal reached in Geneva last month, told the Senate banking committee that any additional sanctions passed by Congress risked strangling a potentially historic diplomatic accord in its crib.

“Now is not the time to introduce new sanctions, because doing so could risk derailing the promising first step,” Sherman testified, arguing that the administration is “prepared to ramp up sanctions” with its allies should Iran violate the terms of the current deal, or should a comprehensive nuclear deal prove unattainable.

Cohen told the panel to expect “mounting sanctions pressure” on Iran even while the negotiations continue over the next six months, citing Thursday’s new designations as evidence.

“No one should doubt our resolve to continue to hold accountable those involved in illicit conduct,” Cohen said. “Iran is still off limits.”

US officials assured a Thursday morning conference call that the Iranians understand that sanctions pressure will actually escalate during the next six months, arguing that the new penalties will not jeopardize the deal. They strongly denied any link between the new penalties and the effort to keep Congress from passing new sanctions.

Senator Tim Johnson, the Democratic chairman of the banking committee, said a pause on new sanctions was reasonable to give diplomacy breathing room, since “existing sanctions will continue to bite, and bite hard.”

“I know this committee is for diplomacy,” Sherman said, softening a White House argument from November that the alternative to the administration’s course is an endorsement of war.

Legislators from both parties have exhibited deep skepticism, occasionally bordering on outright hostility, that a meaningful nuclear deal with Iran is possible. They fear that, as Israeli prime minister Binyamin Netanyahu has argued, the Geneva deal – which expires in six months – heralds the end of sanctions pressure and an inevitable Iranian nuclear weapon.

“Iran cannot be allowed even the possibility of nuclear weaponization,” said Senator Mike Crapo, the top Republican on the banking committee, who implored the administration to vigorously enforce existing sanctions even as it negotiates with Iran.

The Obama administration has responded with a furious lobbying push, sending secretary of state John Kerry to Capitol Hill to sell the deal, which he continued as recently as Wednesday in a closed-door Senate briefing. From the administration’s perspective, Iran has received nothing but up to $7bn in relief from sanctions, which Cohen said was “economically insignificant”. The administration argues that the sanctions architecture will remain in place if a deal is not struck by mid-2014, and which provides the best chance at averting a new military crisis in the Middle East.

The Senate banking committee gave the administration a crucial break this week by announcing it will hold off on passing new sanctions legislation this year.

Sanctions supporters already began to look past the December legislative calendar and predicted a drive to punish Iran in the new year, even as the US and its allies in Britain, France, China, Russia and Germany seek a permanent deal to prevent Iran from going nuclear.

“Even if Congress doesn’t pass sanctions before the end of the year, one can be confident Congress will be back in January, and this effort to pass new sanctions undoubtedly will continue to dominate the discussion,” said Mark Dubowitz of the Foundation for the Defense of Democracies, one of the leading proponents of deepening the sanctions regime.

Nor is the battle likely to end if a deal is reached. Iran, whose economy has lost an estimated $120bn from international sanctions over the past few years and faces an estimated 35% unemployment rate, will want permanent relief from a sanctions regime that has left it isolated on everything from banking to oil sales to the automotive industry.

Such a deal would require the administration to compel Congress to repeal existing sanctions, beyond the president’s executive leeway not to enforce them. If so, that will test the administration’s ability to persuade a recalcitrant and often hostile Congress to do something historic – not only open the door to a potential rapproachement with a deeply disliked adversary, but provide Obama with an achievement for his presidential legacy.

In their testimony, Sherman and Cohen focused instead on the pressing matter of convincing Congress to give their immediate diplomatic efforts time to work, arguing that passing new sanctions would jeopardize the international sanctions coalition that have caused Iran’s economy to contract five percent in 2013.

Other nations will say “wait a minute, you’re changing the rules,” Sherman said, meaning, Cohen added, “the effectiveness of our sanctions will be weakened.”

Senator Bob Corker of Tennessee said that the Senate was engaged in a kind of legislative kabuki over Iran sanctions rather than meaningfully passing anything soon – but still warned of the consequences of a permanent nuclear deal legitimizing Iran in the eyes of the world.

“We’re going through a rope-a-dope and we’re not actually going to do anything,” Corker said. “But once you begin loosening sanctions and people see Iran becoming not a rogue country … there is a rush, as Senator Reed mentioned, to do business with them.”