The outline of the U.S. offer to Iran in their high-stakes negotiations over Tehran's nuclear program is starting to become clearer.

The preliminary U.S. proposal came as a confidential U.N. report described an Iranian initiative that had multiplied Tehran’s stockpile of near-bomb-grade uranium.

"Iran is drafting a negative response to the U.S. proposal, which could be interpreted as a rejection of the U.S. offer," the senior diplomat, who is close to Iran's negotiating team, told Reuters. The U.