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 visit comes at a time of escalating tension between Tehran and the West over Iran's nuclear activities.
The director general of the IAEA, Yukiya Amano, says Iran needs to engage and answer the agency's questions.
Iran denies that it is trying to develop a nuclear weapon.
The head of the IAEA team said they hoped to "resolve all the outstanding issues with Iran" over its nuclear programme.
"In particular we hope that Iran will engage with us on our concerns regarding the possible military dimensions of Iran's nuclear programme," IAEA Deputy Director General Herman Nackaerts said before leaving for Iran from Vienna's airport.
Iran's 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 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".
Since then the European Union and the United States have introduced a series of sanctions against Iran, including measures targeting the country's lucrative oil industry.
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 1 July.
That timing was intended to give some EU countries time to switch to alternative sources of oil, but Iranian lawmakers are now considering stopping exports to Europe within days, a move likely to drive up fuel prices.
Iran has also threatened to close the Strait of Hormuz at the entrance to the Gulf, through which more than 20% of the world's traded oil passes.
The United States has said it will use force if necessary to keep the shipping lane open, raising the prospect of a confrontation with Iran.