The U.N. nuclear watchdog says Iran has further increased its stockpile of uranium enriched to near weapons-grade levels.

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.

The United States has sent its first nuclear deal proposal to Iran since negotiations began in April, a move that comes after a U.N. nuclear watchdog report indicated the Middle Eastern country significantly increased its uranium enrichment over a three-month span.