Diplomats say Iran continues to resist efforts to crimp nuclear activities that could be used to make weapons, with the clock ticking down on a July 20 target date for a deal.
As experts for both sides meet Monday, two diplomats say that a draft agreement still contains large blanks, reflecting disagreement on constraints Iran can accept in exchange for an end to sanctions.
The biggest hurdle remains uranium enrichment, which can make reactor fuel but also the core of a nuclear weapon.
Tehran insists it does not want such arms and has long demanded it be allowed to run up to 50,000 enriching centrifuges. The U.S. insists on only a small fraction of that number.
The diplomats demanded anonymity because they are not authorized to discuss the confidential negotiations.