Baroness Ashton arrived in Tehran on Saturday for a two-day visit, the first by an EU foreign policy chief since 2008, during which she said a wide range of issues, including bilateral ties and regional conflicts, would be discussed.
A long-lasting nuclear deal with Iran would help put an end to years of hostility between Iran and the West, ease the danger of a new war in the Middle East, and open up vast new possibilities for Western businesses.
"The interim deal is really important but not as important as a comprehensive nuclear agreement, which is difficult and challenging," Baroness Ashton told a joint news conference with Iranian Foreign Minister Mohammad Javad Zarif.
Iran and six world powers, represented by Baroness Ashton, struck a deal in November under which Tehran curbed some nuclear activities for six months in return for limited sanctions relief to allow time for a long-term agreement to be hammered out.
The deal, aimed at ending a decade-old dispute over Tehran's nuclear activities, took effect on January 20.
Iran has repeatedly rejected allegations by Western countries it is seeking a nuclear weapons capability, saying its nuclear work is for power generation and medical purposes.
The aim of the talks for the US and its European allies is to extend the "breakout time" Iran would need to produce enough fissile material for a nuclear bomb and to make such a move easier to detect.
Iran has won limited relief of sanctions in exchange for curbing its most sensitive nuclear work under the deal but wants an end to US, EU and UN sanctions that have severely limited its economy.
Mr Zarif said: "Iran is determined to reach an agreement. We have shown goodwill. Now it is up to the other party to show the same goodwill. "