Inspectors from the International Atomic Energy Agency (IAEA) have begun a three-day visit to Iran, to try to determine the purpose of the country's nuclear programme.
The BBC reports the visit comes at a time of escalating tension between Tehran and the West over Iran's nuclear activities.
The IAEA says Iran needs to engage and answer its questions.
Iran's ambassador to the IAEA says the inspection is aimed at foiling enemy plots and will prove the peaceful nature of Iran's nuclear work.
Tehran denies it is trying to develop a nuclear weapon.
The dispute over Iran's nuclear activities has intensified since an IAEA report in November expressed serious concerns about the possible military dimensions of the programme.
The agency said it had information suggesting Iran had carried out tests "relevant to the development of a nuclear explosive device".