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.
"I think this interim agreement is really important, but not as important as the comprehensive agreement that we are currently engaged in. Difficult, challenging, there's no guarantee we'll succeed," Ms Ashton said.
Mr Zarif said: "Iran will only accept a solution that is respectful, that respects the rights of the Iranian people.
"At the same time Iran finds it in its own interest to make sure that there are no ambiguities about Iran's intentions, because we have no intention to seek nuclear weapons."
The two said they had also discussed fighting terrorism, drug trafficking, and conflicts in Afghanistan and Syria. Iran is a key ally of Syrian president Bashar Assad.
Under the historic deal, Iran agreed to halt its 20% enrichment programme, but will continue enrichment up to 5%. It also will convert half of its stockpile of 20% enriched uranium to oxide, and dilute the remaining half to 5%. Enrichment to 20% is a possible pathway to nuclear arms.
The West suspects Iran's nuclear programme has a military dimension. Iran denies the charge, saying its nuclear activities are aimed at peaceful purposes like power generation and medical treatment.