Iran has not expanded its nuclear facilities in the last three months, a UN atomic watchdog report said Thursday, in a possible confidence-building measure by Tehran before talks next week.
The International Atomic Energy Agency said only four new uranium enrichment centrifuges were operating at Iran's Natanz plant and no additional machines at the Fordo facility.
It added that Iran has also not begun operating any new-generation IR-2M centrifuges and that "no... major components" had been installed at a reactor being built at Arak.
The faster IR-2M centrifuges are of concern to the international community because in theory they shorten the time needed by Iran to produce enough weapons-grade uranium for a nuclear bomb.
The so-called IR-40 reactor at Arak is a worry because it could provide Iran with plutonium, an alternative to uranium for a nuclear weapon, once it has been up and running for 12 to 18 months.
Iran says its nuclear programme is peaceful.
"There is absolutely no technical reason for this (freeze)," said a senior diplomat familiar with the IAEA's activities in Iran.
"This is clearly a choice," he said on condition of anonymity.
"They haven't introduced uranium to any new centrifuges in the last three months. They have more or less frozen construction of the IR-40, major components are still missing."
The quarterly IAEA report, the first since Iranian President Hassan Rouhani took office in August, was released ahead of a new round of talks between Iran and world powers in Geneva next week.
The last report three months ago showed that Iran had added 1,861 IR-1 centrifuges at Natanz. Three months before that it put 886 more machines on line and three months before that 2,255.
Three gruelling days of talks on Iran's nuclear programme that involved US Secretary of State John Kerry and other foreign ministers ended with no agreement in the early hours of Sunday morning.
The United States, China, Russia, Britain, France and Germany (the so-called "P5+1") want Iran to freeze the most sensitive parts of its nuclear programme.
This includes the enrichment of uranium to fissile purities of 20 percent, close to weapons-grade, and a halt to construction at Arak.
In return Iran wants UN and Western sanctions that have been hammering the Islamic republic's economy to be eased soon, and its "right" to enrich uranium recognised.