EU Energy Commissioner Miguel Arias Canete arrived in Tehran on Saturday to present plans for continuing oil and gas purchases and protect European companies despite renewed US sanctions on Iran.
Canete was due to meet Iran's Atomic Energy Organisation head Ali Akbar Salehi, Environment Minister Isa Kalantari and Oil Minister Bijan Namdar Zanganeh on Saturday.
Talks with Foreign Minister Mohammad Javad Zarif were scheduled for Sunday.
He is the first Western official to visit Iran since President Donald Trump announced he was pulling the US out of the 2015 nuclear deal, and would reimpose sanctions, that are expected to hit European firms hardest.
Although European leaders have made firm vows to maintain the deal, many of their companies -- including France's Total and Holland's Maersk -- have already said it will be impossible to stay in Iran once US sanctions are fully reimposed over the next six months.
Iran's trade with the European Union is around 20 billion euros, evenly split between imports and exports.
The vast majority of EU purchases from Iran -- 90 percent -- is oil purchases, going primarily to Spain, France, Italy, Greece, the Netherlands and Germany.
Iran, which has the world's fourth-biggest oil reserves, produces some 3.8 million barrels of oil per day, 70 percent of which goes to China and other Asian countries, and 20 percent to Europe.
It also has the second-biggest gas reserves in the world, but limited infrastructure means little is exported.
Russia and China -- the other parties to the nuclear deal -- have also vowed to maintain trade with Iran, and because they are less exposed to US markets, are less vulnerable to economic pressure from Washington.