A team of inspectors from the United Nations nuclear agency has begun a three-day visit to Iran to try to determine the purpose of the country's nuclear programme.
The visit by the International Atomic Energy Agency comes at a time of heightened tension between Iran and the West.
The European Union approved tough sanctions against Iran on Monday in an attempt to stop Tehran's alleged pursuit of nuclear weapons.
The head of the IAEA team said it hoped to "resolve all the outstanding issues with Iran" over its nuclear programme.
Iran denies that it is trying to develop a nuclear weapon and its ambassador to the IAEA, Ali Asgar Soltaniyeh, said the inspection was aimed at foiling enemy plots and will prove the peaceful nature of Iran's nuclear work.
The EU sanctions prohibit the import, purchase and transport of Iranian crude oil and petroleum products as well as related finance and insurance. All existing contracts will have to be phased out by July.
That timing was intended to give some EU countries time to switch to alternative sources of oil.
However, Iran's parliament is due to debate a bill that would cut off oil supplies to the EU within days, a move likely to drive up fuel prices.