The United States and Iran have begun a third round of nuclear talks, entering what experts describe as a more difficult phase of technical negotiations this weekend as Washington lays out its conditions.

Iran is ringing two deeply buried tunnel complexes with a massive security perimeter linked to its main nuclear complex, a report said Wednesday, amid U.S. and Israeli threats of attack.

Michael Anton, the State Department’s head of policy planning, leads the US expert-level delegation, while deputy foreign ministers Kazem Gharibabadi and Majid Takht-Ravanchi will lead Tehran’s, according to Iran’s Tasnim news agency.