Diplomats at the United Nations Nuclear Agency say Iran has started small-scale uranium enrichment, a process that produces fuel that can be used in nuclear weapons and reactors.
The International Atomic Energy Agency officials, speaking on condition of anonymity, say uranium gas has been fed into some machines.
Iran had warned it would resume large-scale enrichment activities after the IAEA referred it to the U.N. Security Council for possible sanctions over its nuclear program.
Iran also further postponed talks set for this week with Russia about a proposal to process uranium on Russian soil for use in Iran's nuclear plants.
Russia has been pushing for the plan as a way to ease international concerns that Iran might be aiming to produce weapons-grade uranium. Iran says its nuclear program is only intended to generate electricity.