Iran has come within roughly a month of having enough material to fuel a single nuclear weapon, crossing a threshold that may raise pressure on the United States and its allies to improve the terms of a potential deal to restore the 2015 nuclear agreement.

Experts studying new data contained in reports last week by the International Atomic Energy Agency, the United Nations’ atomic inspection group, say that by enriching nuclear fuel in recent months to near bomb-grade levels, Tehran has gained the capability to produce the fuel needed for a single nuclear warhead within a month or so, under the most extreme timeline. Federal officials who have seen classified estimates are prevented from discussing official assessments but conceded in background conversations that they believed it would take Iran only a few months.

Manufacturing an actual warhead — one that could fit atop an Iranian missile and survive the fiery re-entry into the atmosphere, a technology the Iranians were actively studying 20 years ago — would take considerably longer. Iran continues to insist it has no desire for a nuclear arsenal.