MUSCAT, Oman —Iran and the United States held their first technical-level talks on Tehran’s rapidly advancing nuclear program Saturday, a more complex phase of their high-stakes negotiations.

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.

The talks seek to limit Iran's nuclear program in exchange for the lifting of some of the crushing economic sanctions the U.S. has imposed on the Islamic Republic.