The White House declined Friday to renew its previous prediction that a Senate bill slapping new sanctions on Iran, which it fears could scupper nuclear talks, would not pass the Senate.

President Barack Obama's spokesman Jay Carney did deliver a new warning, however, that the bill would be detrimental to the drive to resolve the nuclear showdown with Tehran short of war.

"I think that we remain hopeful that Congress will not pass such a sanctions bill because of the negative effect that would have on the ongoing negotiations and the potential to resolve this peacefully," Carney said.

"But I'm not going to make legislative predictions."

Carney's comments appeared to hint at an eroding of the administration's position on the bipartisan legislation since December, when he threatened Obama would veto the bill if it passed.

"We don't think it will be enacted. If it were enacted, the president would veto it," Carney said of the legislation on December 19.

According to Senate procedure, a bill technically becomes an Act when it has passed one chamber of Congress.

But it does not become law until the president signs it or until both the House of Representatives and the Senate have produced the necessary two-thirds vote to override a veto.

Obama and senior aides have repeatedly urged a bipartisan group of senators not to pass more sanctions on Iran, fearing they could undermine Tehran's negotiating team with conservatives back home and prompt Tehran to walk away from the table.

Lawmakers who support the bill say tough sanctions brought Iran to the negotiating table and stiffer measures would increase Obama's leverage in talks between the Islamic Republic and the P5+1 group of world powers.

The latest wrangling over sanctions in Washington came as negotiators agreed on how to implement a six-month interim deal on curtailing Iran's nuclear program in the latest round of talks in Geneva.

The deal, meant to provide time and space to negotiate a permanent pact, will now be sent to respective national capitals before it can be put into force.

Obama has insisted that Washington must test Iranian President Hassan Rouhani's willingness to honor a pledge to seek a peaceful nuclear deal, despite opposition from many hawks on Capitol Hill and deep reservations by America's closest Middle Eastern ally, Israel.

Action by Congress, he argues, would be an unnecessary impediment to the talks. Obama stresses however that he has not taken the option of using military force to thwart Iran's nuclear ambitions if diplomacy fails.

Carney said the new sanctions would have the opposite effect to the one intended by key sponsors, Democratic Senate Foreign Relations Committee Chairman Robert Menendez and Republican Senator Mark Kirk.

"It could, if they were to do it, actually weaken the sanctions structure that's in place by undermining faith among our international partners and providing Iran the opportunity to say that we have been negotiating in bad faith," Carney said.

In a Washington Post editorial, Menendez described the initiative as a "diplomatic insurance policy" against Iran.

He said his bill would impose immediate extra sanctions on Iran if it became necessary but would not come into force while "good faith" negotiations were under way.

"Should Iran breach this agreement or fail to negotiate in good faith, the penalties it would face are severe," he wrote.

New sanctions would further target Iranian petroleum products and the mining, engineering and construction sectors.

The White House counters that if Iran reneges on the interim deal, which provides for limited relief on sanctions in return for a curbing of aspects of the nuclear program, it would support the swift passage of new sanctions.

It remains unclear when the sanctions bill could be brought up in the Senate. But several reports suggest that support is growing for the measure despite the administration's intense lobbying effort.

Ten key Democratic Party committee chairs in the Senate last year demanded that the drive for new sanctions be put on hold, backing the administration's arguments.

The White House first warned US lawmakers back in November that tightening sanctions on Iran could box the United States into a "march to war" and derail a diplomatic push to limit Tehran's atomic program.

Bernadette Meehan, a National Security Council spokeswoman, delivered a similar warning.

"If certain members of Congress want the United States to take military action, they should be up front with the American public and say so," Meehan told The Huffington Post news website.

Western nations have long suspected Iran is pursuing nuclear weapons alongside its civilian program, allegations denied by Tehran, which insists its nuclear activities are entirely peaceful.