Iran has yet to suspend the enrichment of uranium, as it agreed to do last week in talks on its nuclear programme.
Tehran denies it has a nuclear weapons programme
The foreign ministry said officials were still studying how to halt the nuclear fuel cycle.
Tehran promised to halt its enrichment process and allow tougher UN inspections of its nuclear facilities during talks with three EU ministers.
Iran faces a 31 October deadline to prove to the UN atomic watchdog that it is not trying to build nuclear arms.
The UN agency - the International Atomic Energy Agency (IAEA) - is due to review Iranian compliance with its demands on 20 November.
During Tuesday's talks with the foreign ministers of France, Germany and the UK, the Iranian Government agreed to halt "temporarily" its uranium enrichment programme, which it says it has peaceful purposes.
In return, the European ministers recognised Iran's right "to the peaceful use of nuclear energy".
But on Sunday, foreign ministry spokesman Hamid Reza Asefi said officials were still studying "the modalities of a suspension".
Tehran also agreed to sign an additional protocol of the Nuclear Non-Proliferation Treaty (NPT), allowing UN inspectors to carry out spot checks of Iranian facilities.
On Thursday, the IAEA said it had received documents from Iran clarifying its past nuclear activities.
Agency experts arrived in Tehran on Sunday, to try to verify whether that account is accurate.
Five IAEA officials will conduct interviews and inspections and take samples where necessary.
They will be focusing on processes of uranium conversion, laser enrichment and centrifuges where traces of highly enriched weapons grade uranium were found earlier, the BBC's Jim Muir reports from Tehran.
Their findings will provide the basis for the IAEA chief's report back to his board on 20 November.