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.