EU foreign policy chief Catherine Ashton said there is "no guarantee" that talks with Iran on its disputed nuclear programme will lead to a comprehensive agreement.
"This interim agreement is really important but not as important as a comprehensive agreement (which is)... difficult, challenging, and there is no guarantee that we will succeed," she told a joint news conference with Iranian Foreign Minister Mohammad Javad Zarif.
Ms Ashton has travelled to Tehran to discuss a range of issues including Iran's disputed nuclear programme, before another round of talks between Iran and world powers.
During her two-day visit, her first to Iran and the first by an EU foreign policy chief since 2008, bilateral, regional and international issues will be discussed, the semi-official Fars news agency reported.
Iranian state media and newspapers have given high importance to Ms Ashton's visit, with some reformist media calling it "an achievement" for President Hassan Rouhani.
Iran and six world powers, represented by Ms Ashton, reached an interim deal in Geneva in November aimed at ending a decade-old dispute over Tehran's nuclear activities.
Seeking to build on the interim accord, which took effect on 20 January, Iran and the six powers aim to hammer out a final deal by late July.
The next round of high-level talks will be held in Vienna on 17 March.
The six powers' overarching goal is to extend the time Iran would need to make enough fissile material and assemble equipment 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 Geneva deal but demands a full halt to all EU, US and international sanctions to end years of isolation and revive its battered economy.