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.