The assessment comes as the Obama administration tries to persuade Congress not to proceed with a bill that would toughen sanctions against the Islamic republic, USA Today reported Thursday.
"Shortening breakout times have implications for any negotiation with Iran," a report by the Institute for Science and International Security said. "An essential finding is that they are currently too short and shortening further."
David Albright, the institute's president and a former inspector for the U.N. International Atomic Energy Agency, said the estimate means that Iran would have to eliminate more than half of its 19,000 centrifuges to extend the time it would take to build a bomb to six months.
In the report, Albright said negotiations with Iran should focus on "breakout" times, the time needed to convert low-enriched uranium to weapons-grade.
The Obama administration has said Iran is probably a year away from having enough enriched uranium to make a bomb.
USA Today said the National Security Council and the State Department did not respond to requests for comment.
Recently elected Iranian President Hassan Rouhani has said his country isn't interested in nuclear weapons and that its nuclear activities are for peaceful purposes.
The Senate Banking Committee is considering legislation that would tighten sanctions on Iran. Committee member Mark Kirk, R-Ill., said the report indicates Iran is expanding its nuclear capabilities under the cover of negotiations.
"The Senate should move forward immediately with a new round of sanctions to prevent Iran from acquiring an undetectable breakout capability," he said.
The Obama administration has said consideration of any new sanctions should wait while current negotiations scheduled to officially resume next month move ahead.
The White House said Thursday it still would consult with Congress "so that any congressional action is aligned with our negotiating strategy as we move forward," said Caitlin Hayden, a National Security Council spokeswoman.
USA Today reported NSC said the intelligence community maintains several assessments on possible timetables for Iran to produce enough weapons-grade uranium for a weapon or a nuclear device that can be tested.