Iran is poised to reject a US proposal to end a decades-long nuclear dispute, an Iranian diplomat said, slamming it as a "non-starter" that fails to address Tehran's interests.

The United States presented its first formal proposal to Iran for elements of a nuclear deal on Saturday, just hours after U.N. inspectors reported a major surge over the past three months in the size of 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.