LAUSANNE, Switzerland -- With a negotiating deadline just two days away, Iranian officials on Sunday backed away from a critical element of a proposed nuclear agreement, saying they are no longer willing to ship their atomic fuel out of the country.
For months, Iran tentatively agreed that it would send a large portion of its stockpile of uranium to Russia, where it would not be accessible for use in any future weapons program. But on Sunday, Iran's deputy foreign minister made a surprise comment to Iranian reporters, ruling out an agreement that involved giving up a stockpile that Iran has spent years and billions of dollars to amass.
"The export of stocks of enriched uranium is not in our program, and we do not intend sending them abroad," the official, Abbas Araqchi, told the Iranian media, according to Agence France-Presse. "There is no question of sending the stocks abroad."
Western officials confirmed that Iran was balking at shipping the fuel out but insisted that there were other ways of dealing with the material. Chief among those options, they said, was blending it into a more diluted form.
Depending on the technical details, that could make the process of enriching it for military use far more lengthy or perhaps nearly impossible.
Nonetheless, the revelation that Iran is insisting on retaining the fuel could raise a potential obstacle at a critical time in the talks. And for critics of the emerging deal in Congress, in Israel and in Sunni Arab nations like Saudi Arabia, the prospect of leaving large amounts of nuclear fuel in Iran, in any form, is bound to intensify their already substantial political opposition.
If an accord allowing Iran to retain the fuel is reached, the Obama administration is expected to argue that it would not constitute a serious risk, particularly if it is regularly inspected. So far under an interim agreement negotiated in 2013, Iran has complied fully with a rigorous inspection process for the stockpiles of its fuel, the International Atomic Energy Agency has said.
But the development could give opponents another reason to object, adding it to a list of what they call concessions made by an administration in search of an agreement. If Iran ever bars the inspectors from the country, as North Korea did a dozen years ago, the international community would have no assurance about the fate of the fuel.
Nor has Iran answered long-standing questions about its suspected nuclear design and testing of components that could be used to detonate a warhead.
Ray Takeyh, a senior fellow at the Council on Foreign Relations who has been critical of the emerging accord, said the development raised serious questions about a possible deal.
"The viability of this agreement as a reliable arms control accord is diminished by this," Takeyh said. "One of the core administration arguments has been that the uranium would be shipped abroad as a confidence building measure."
On the assumption that Iran's uranium stockpile would be small, the U.S. and its negotiating partners had been moving toward an agreement that would allow Iran to retain roughly 6,000 centrifuges in operation. It is not clear how much that might change if the fuel, even in diluted form, remains in the country.
If the fuel had been shipped to Russia, the plan called for Moscow to convert it into specialized fuel rods for the Bushehr nuclear power plant, Iran's only commercial reactor. Once it was converted into fuel rods, it would have been extremely difficult for Iran to use the material to make a nuclear weapon.
It is not clear what form the fuel would take if it remained on Iranian territory.
The disclosure also adds a new element to the growing debate over whether the proposed agreement would meet President Barack Obama's oft-stated assurance that the world would have at least a year's warning if Iran raced for a bomb - what experts call "breakout time."
The argument over warning time, which was accelerated by a skeptical paper published over the weekend by the former chief inspector of the International Atomic Energy Agency, offered a taste of the kind of arguments already taking shape in Congress.
On Sunday, Republican leaders made it clear they would press for more sanctions against Iran if no agreement is reached here by Tuesday. In an interview with CNN, House Speaker John A. Boehner expressed doubts about a potential agreement on Iran's nuclear program.
"We have got a regime that's never quite kept their word about anything," he said. "I just don't understand why we would sign an agreement with a group of people who, in my opinion, have no intention of keeping their word."
With pressure mounting to settle on the main parameters of an accord, negotiators were still divided on how fast United Nations' and others' sanctions on Iran might be lifted. Important differences remained on what kind of research and development Iran could carry out on new types of centrifuges during the last five years of what is intended to be a 15-year agreement.
There was a clear sense that the talks were approaching a pivotal moment as the foreign ministers from other world powers joined Secretary of State John Kerry in an effort to reach the outlines of a deal by a midnight Tuesday deadline.
"We are not there yet," said one Western official who, like others in this article, declined to be identified because he was discussing diplomatic deliberations. "There are lots of pieces floating around."
Yet even if a deal was reached by late Tuesday, U.S. negotiators made clear that this was just an interim step and that any final agreement would require months of negotiations over what were once called "technical agreements" but are now clearly the source of continuing disagreement.
That calculation over "breakout time" is so complex that experts from Britain, France, Germany and Israel all have somewhat slightly different calculations than those of experts from the United States.
The debate over breakout time intensified when Olli Heinonen, who ran inspections for the IAEA before moving to Harvard several years ago, published a paper on Saturday concluding that, based on leaked estimates that Iran would operate roughly 6,500 centrifuges, "a breakout time of between seven and eight months would still be possible."
A senior Obama administration official here said that while he did not dispute Heinonen's figures, the former inspector had conducted a textbook calculation rather than examining the real-life conditions at Iran's facilities.
Like other countries, Iran loses some of its nuclear material every time it is changed from a gas to a solid, and its machinery, the evidence shows, does not run at perfect efficiency. The official said that the United States had created a measure based on what U.S. officials have called the "fastest reasonable" estimate of how long Iran would take to produce a weapon.
Some experts outside government say the U.S. assumptions are reasonable, and perhaps even generous to the Iranians - who have taken 20 years to get to this point, far longer than it took other programs, including in North Korea and Pakistan, to produce bomb-grade material.
But the emergence of competing estimates could pose a political problem for Obama, who has made breakout time the paramount measure for a potential agreement.
Parts of the agreement have begun to leak out and reflect the balancing act underway: An effort by the U.S. and the other five powers here to cripple Iran's ability to produce enough nuclear material for a weapon for at least 10 years, while letting the Iranians preserve a narrative that they are not dismantling major facilities, or giving in to U.S. pressure.
For example, a deep underground facility at Fordow - exposed in 2009 - would likely be converted to make medical isotopes. That means it would not be used for enriching uranium.
But several hundred centrifuges might still be spinning there - the facility now has about 3,000 - and that fact alone, U.S. officials acknowledge, could provide fodder to opponents of the deal.
Reporting was contributed by Thomas Erdbrink from Tehran, and Andrew Siddons from Washington.