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 U.N. atomic agency reports that Iran hasn’t cooperated with a nuclear probe, raising the prospect of a snapback of sanctions on Tehran.