Iran may be poised to expand its nuclear programme at an underground site near the city of Qom, a Vienna-based diplomat has told the BBC.
It appears to be ready to install thousands of new-generation centrifuges at the fortified underground plant, the diplomat said.
They could speed up the production of enriched uranium - required for both power generation and nuclear weapons.
The United Nations nuclear watchdog, the IAEA, has not commented.
Its inspectors are due to visit Tehran this week for another round of talks on Iran's nuclear activities, after they were denied access to certain nuclear sites and scientists on a visit last month.
Iran says its nuclear work is for purely peaceful purposes, but Western countries express fears that Iran is secretly trying to develop a nuclear bomb.