European Union foreign policy chief Catherine Ashton on Sunday said there is "no guarantee" that nuclear talks with Iran will succeed beyond an interim 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," AFP quoted the EU diplomat as saying during a news conference in Tehran with Iranian Foreign Minister Mohammad Javad Zarif.
Ashton arrived in Tehran on Saturday to discuss issues including Iran's disputed nuclear program, before another round of talks between Iran and world powers, Iranian media reported.
During Ashton's 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 Ashton's visit, with some reformist media calling it "an achievement" for pragmatist president Hassan Rouhani, who won a surprise election in June on a platform of more openness towards the West.
"Naturally, the nuclear issue will be discussed during Mrs. Ashton's visit," Iranian state television showed Deputy Foreign Minister Abbas Araqchi saying.
Seeking to build on the interim accord, which took effect on January 20, Iran and the six powers aim to hammer out a final deal by late July. The next round of the high-level talks
between Iran and the P5+1 will be held in Vienna on March 17.
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.