Accused by Western nations of running a covert atomic weapons programme, Iran had frozen all work at Isfahan late last year under a deal with France, Britain and Germany. But it resumed work at the plant in August, prompting the EU's three biggest powers to suspend talks with the Islamic republic.
"Beginning next week, the Iranians will start a new phase of uranium conversion at Isfahan. They will begin feeding a new batch of uranium into the plant," a European diplomat familiar with the result of inspections by the UN nuclear watchdog told Reuters on condition of anonymity.
Iran denies wanting nuclear weapons, insisting its atomic ambitions are limited to the peaceful generation of electricity. However, it has acknowledged concealing many nuclear activities from the International Atomic Energy Agency (IAEA) for 18 years.
A Western diplomat close to the IAEA said he was unable to provide details on how much uranium would be fed into the plant.
Speaking at Harvard University's John F. Kennedy School of Government on Tuesday, IAEA Director-General ElBaradei said the IAEA has been gaining access to military facilities inside Iran - sites the agency has had great difficulty in visiting.