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If the talks produce a deal to freeze Iran’s nuclear efforts, negotiators will pursue a more comprehensive agreement that would ensure that Tehran’s program is solely for civilian purposes. Iran would get some sanctions relief under such a first-step deal, without any easing of the harshest measures — those crippling its ability to sell oil, its main revenue maker.

Iran has suggested it could curb its highest-known level of enrichment — at 20 percent — in a possible deal that could ease the U.S.-led economic sanctions.

But Iranian leaders have made clear that their country will not consider giving up its ability to make nuclear fuel — the centerpiece of the talks since the same process used to make reactor stock can be used to make weapons-grade material.

Details of sanctions relief being discussed have not been revealed. But a member of the U.S. Congress and legislative aides on Wednesday put the figure at $6 billion to $10 billion, based on what they said were estimates from the U.S. administration.

The aides and the member of Congress demanded anonymity because they weren’t authorized to divulge the estimate publicly.