Beefing up international monitoring of Iran's nuclear work could become the biggest stumbling block to a final accord between Tehran and major powers, despite a preliminary deal reached a week ago.
As part of that deal, Iran and the powers agreed that United Nations inspectors would have "enhanced" access to remaining nuclear activity in Iran, where they already monitor key sites.
But details on exactly what kind of access the inspectors will have were left for the final stage of talks, posing a major challenge for negotiators on a complex and logistically challenging issue that is highly delicate for Iran's leaders.