Uranium enrichment is at the center of talks in Geneva, Switzerland, between representatives of the United States, its diplomatic partners and Iran, CNN reported Thursday.
Iran responded quickly.
"Enrichment is our red line and its suspension is unacceptable," the Iranian state-run Fars News Agency said in a tweet it attributed to Deputy Foreign Minister Abbas Araqchi, Iran's lead negotiator.
Araqchi did not respond later when asked about the issue of uranium enrichment.
Mann said the talks were entering "a serious phase."
Iran has processed about 10 metric tons (11 tons) of uranium with 5 percent purity, the level needed for nuclear power plants, the International Atomic Energy Agency said in August.
In the same report, the IAEA said Iran had processed about 185 kilograms (400 pounds) of uranium with 20 percent purity, the level needed for atomic bombs. However, the amount is insufficient to build a weapon.