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WASHINGTON — The Obama administration, attempting to salvage a faltering nuclear deal with Iran, has told Iran’s leaders in back-channel messages that it is willing to allow the country to send its stockpile of enriched uranium to any of several nations, including Turkey, for temporary safekeeping, according to administration officials and diplomats involved in the exchanges.

Mohamed ElBaradei, the director general of the International Atomic Energy Agency.

But the overtures, made through the International Atomic Energy Agency over the past two weeks, have all been ignored, the officials said. Instead, they said, the Iranians have revived an old counterproposal: that international arms inspectors take custody of much of Iran’s fuel, but keep it on...