Top EU officials including foreign policy chief Federica Mogherini arrived in Iran on Saturday to improve cooperation with the once-pariah state.
The agenda was reported to cover new economic ties after the end of sanctions linked to Tehran's nuclear programme, the security situation in Syria, the opening of a EU mission, and more contentious issues including Iran's use of capital punishment, and refugee flows.
"This visit is an important step towards building cooperative relations between the European Union and Iran on the areas where there is mutual interest and to have frank discussions on the issues where differences remain," Mogherini said ahead of the trip.
She traveling with EU commissioners in charge of industry, transport, energy, research, science, education, culture, humanitarian aid and the environment.
Talks with Iranian Foreign Minister Mohammad Javad Zarif and other ministers are foreseen in the capital to discuss areas ranging from economic relations and civil nuclear cooperation to drugs and migration, according to the European Commission, the EU's executive.
Iranian President Hassan Rowhani was called away at short notive to Ankara and would be unable to meet the EU delegation.
Europe is struggling to cope with an influx of migrants and asylum seekers, including Afghan migrants from Iran. The country hosts around 3 million refugees from neighbouring Afghanistan.
Human rights issues will also be on the agenda. The EU still has sanctions in place against Iran over human rights concerns. But the EU did not request a meeting with Iranian civil society representatives because of the shortness of the visit, a senior EU official said on condition of anonymity.
Relations between Iran and the EU have been thawing since Tehran signed a deal last year with six global powers to curb its nuclear activities. In return, Iran secured the removal of crippling economic sanctions and re-emerged on the world stage.
"Our objective remains to reassure everyone – particularly in the financial sector – that re-engaging with Iran is possible now," the EU official said.
Tehran is thought to be interested in closer cooperation with the EU on energy and trade issues, notably possible membership in the World Trade Organization. Mogherini is expected to brief the bloc's 28 foreign ministers about the visit on Monday at talks in Brussels.