Washington: A bipartisan group of senators, defying the White House, introduced a bill on Thursday to impose fresh sanctions on Iran if it failed to conclude a nuclear agreement, or stick to the terms of its interim deal, with the US and other major powers.

The Obama administration swiftly condemned the legislation, warning that it could derail negotiations with Iran and that President Barack Obama would veto it if it ever came to his desk. With the Senate about to recess for the Christmas holiday, that is unlikely for the time being.

But the bill, backed by 26 senators evenly divided between Republicans and Democrats, confronts the White House with the prospect of a sustained showdown with Congress during its delicate talks with Iran - talks that Iranian officials said would be scuttled by any new sanctions.

Among those backing the bill are traditional administration allies like Senator Charles E. Schumer, and Senator Bob Casey, as well as dedicated Iran hawks like Senator Robert Menendez, who is chairman of the Senate Foreign Relations Committee.

“Current sanctions brought Iran to the negotiating table, and a credible threat of future sanctions will require Iran to cooperate and act in good faith,” Menendez said in a statement.

The bill would seek to drive Iran’s oil exports down to zero and penalise its engineering, mining and construction industries. But the sanctions would not take effect before the six-month term of the interim deal expires, and they could be deferred for up to another six months, at Obama’s request, if the talks looked promising.

None of that mollified the White House, which lobbied fiercely against the bill, issued its first public veto threat, and clearly seemed worried that the legislation would take on a life of its own in a Congress where sanctioning Iran is one of the few genuinely bipartisan issues.

“At a certain point, they have to understand we mean what we say,” said Benjamin J. Rhodes, a deputy national security adviser. “We just can’t have new sanctions during this period.”

The sanctions debate has deeply divided Democrats, with 10 senators who head committees sending a letter to the Senate majority leader, Senator Harry Reid of Nevada, requesting that he hold off the bill while negotiations are underway. Among those who signed it were Senator Carl Levin, , chairman of the Armed Services Committee and Senator Dianne Feinstein, chairwoman of the Intelligence Committee.

Much now depends on Reid, who helped the White House by refusing to attach the sanctions to a Pentagon funding bill but has expressed sympathy for the concerns of the Israeli government and others about a nuclear deal with Iran.

Two weeks ago, Iran’s foreign minister, Mohammad Javad Zarif, said new sanctions would kill the nuclear talks, saying in Time magazine, “We do not like to negotiate under duress.”

Proponents of sanctions contend, however, that the US called Iran’s bluff last week by imposing penalties on Iranian companies and individuals for evading sanctions. Iran briefly suspended technical talks on the nuclear deal, but has returned to the table in Geneva.

“The American people rightfully distrust Iran’s true intentions, and they deserve an insurance policy to defend against Iranian deception during negotiations,” said Senator Mark S. Kirk, and a lead sponsor of the bill, along with Menendez.

Other supporters argue that having extracted limited relief from sanctions in the interim deal, the Iranian government is unlikely to risk the much greater damage its economy would incur if it were to abandon the nuclear talks over the mere threat of sanctions.

“The cost of walking away would be pretty profound,” said Mark Dubowitz, the executive director for the Foundation for Defence of Democracies, who puts the price of the new sanctions to the Iranian economy at a minimum of $55 billion (Dh202 billion) a year, much of it in lost oil exports.

The administration counters that Congress could quickly enact sanctions if the talks fail, but that to do so now would jeopardise the effort.

“We don’t want to see actions that will proactively undermine American diplomacy,” said the White House spokesman, Jay Carney.

In trying to buttress the administration’s argument against new sanctions, the State Department took the unusual step of stating on Thursday that US intelligence agencies had assessed that such measures would make it more difficult to negotiate a comprehensive nuclear accord with Iran.

“It appears in this case with the introduction of this legislation that they’ve chosen to ignore the assessment of our negotiators and also our intelligence community, which has said that additional sanctions would make this harder,” said Marie Harf, the State Department’s deputy spokeswoman.

“The intelligence community’s December 10, 2013, assessment states that, quote, ‘New sanctions would undermine the prospects for a successful comprehensive nuclear agreement with Iran,’” she said.

It was not clear if this intelligence assessment applied only to sanctions that would take effect during the six-month period when a comprehensive agreement is to negotiated or whether it also applied to legislation that would defer new sanctions for six months and would impose them only if a comprehensive accord was not reached.