Iran could produce enough weapons-grade uranium to build a nuclear bomb in as little as a month, according to a new estimate by one of the USA’s top nuclear experts.

The new assessment comes as the White House invited Senate staffers to a briefing on negotiations with Iran as it is trying to persuade Congress not to go ahead with a bill to stiffen sanctions against Iran.

“Shortening breakout times have implications for any negotiation with Iran,” stated the report by the Institute for Science and International Security. “An essential finding is that they are currently too short and shortening further.”

David Albright, president of the institute 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.