EU foreign policy chief Catherine Ashton said on Sunday 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," Ashton told a joint news conference in Tehran with Iranian Foreign Minister Mohammad Javad Zarif.
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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.
"It is very important that with the support of the people of Iran for the work to going on by the minister and his team and with the support of international community for my work that we should aim to try to succeed," Ashton said.
Western nations and Israel have long suspected Iran is covertly pursuing a nuclear weapons capability alongside its civilian programme, charges denied by Tehran.