The UN nuclear watchdog is said to be voicing “increasing concern” about the possibility of Iran developing nuclear energy for military purposes.
Several news agencies say they have obtained copies of a report from the International Atomic Energy Agency.
The agency is reported as saying that Iran has begun installing machines for higher-grade uranium enrichment in an underground bunker near the holy city of Qom.
The news agencies also say the latest UN report mentions a more advanced model of centrifuge being used to enrich uranium at another research and development facility.
Iran denies its nuclear programme is designed for military purposes.