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 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.

The revelation that Iran is now insisting on retaining the fuel raises a potential roadblock 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 the already substantial political opposition to any accord.

The Obama administration may be able to make a technical argument that the diluted fuel 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.

‘Breakout time’

The disclosure also adds a new element to the growing debate over whether the proposed agreement would meet President 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 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.