Iran nuclear deal 'not guaranteed'
The European Union's foreign policy chief said there is "no guarantee" world powers will reach a final deal with Iran.
Catherine Ashton has been in Tehran for meetings with Iranian officials on negotiations over the country's nuclear programme.
Under an interim deal in November, Iran agreed to limit a key nuclear activity, uranium enrichment, in return for the West easing sanctions.
Negotiations for a final deal are ongoing. Ms Ashton leads the six-nation group - the US, Britain, France, Russia, China and Germany - in talks with Iran.
"There is no guarantee we'll succeed," she told reporters in a joint briefing with Iranian Foreign Minister Mohammad Javad Zarif.
Mr Zarif said Iran will only accept a deal that respects its "rights," a reference to uranium enrichment on its soil.