EU foreign policy chief Catherine Ashton has said there is "no guarantee" that talks with Iran on its disputed nuclear programme will lead to a comprehensive final agreement.
Ashton made the comments on Sunday during her first trip to the Islamic Republic.
"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," Ashton told a joint news conference in Tehran, which was also attended by Iranian Foreign Minister Mohammad Javad Zarif.
Zarif said Iran will only accept a deal that respects its "rights," a reference to uranium enrichment on its soil.
Iran clinched the interim deal in November with the so-called P5+1 - Britain, China, France, Russia, the United States and Germany - under which it agreed to curb its nuclear activities in exchange for sanctions relief.
Negotiators are trying to reach a final accord in the coming months.
Western nations and Israel have long suspected Iran is covertly pursuing a nuclear weapons capability alongside its civilian programme, charges denied by Tehran.
Ashton arrived in Tehran on Saturday night to meet with Iranian leaders, including President Hassan Rouhani.
Iranian media said the visit signalled the normalisation of Iran's relations with Europe after eight years of tension under Rouhani's predecessor, President Mahmoud Ahmadinejad.
Experts of all parties have already met to prepare the way for the next high-level talks in Vienna, Austria, on March 17.