Talks on long-term Iran nuclear deal open in Vienna
Iran has begun formal talks with six world powers in Vienna to try to reach a comprehensive agreement to limit its controversial nuclear programme.
The three-day meeting seeks to build on an interim deal signed in November that saw Iran curb uranium enrichment in return for partial sanctions relief.
Both Iran and the US have downplayed hopes of a quick breakthrough.
The world powers suspect Iran is seeking to develop nuclear weapons, something it vigorously denies.
It insists that its nuclear programme is for peaceful purposes only.
The talks between representatives of Iran and the so-called P5+1 - the US, UK, France, China and Russia plus Germany - are being chaired by the EU's foreign policy chief, Catherine Ashton.
After dinner with Baroness Ashton on Monday, Iranian Foreign Minister Mohammad Javad Zarif said he believed a long-term agreement was possible.
"If all sides enter the talks with the political will, we will be able to reach positive results. But it will take time," he was quoted as saying by the state news agency, Irna.
Mr Zarif's remarks came after Iran's Supreme Leader Ayatollah Ali Khamenei, who has final say in nuclear matters, was pessimistic about the prospects of a long-term deal.
"What our foreign ministry and officials have started will continue and Iran will not violate its commitments... but this will not lead anywhere," he said in a speech in Tehran.
He added: "I am not optimistic about the negotiations. It will not lead anywhere, but I am not opposed either."
A senior US government official also played down expectations, saying it would be a "complicated, difficult and lengthy process".
"It is probably as likely that we won't get an agreement as it is that we will," the official told reporters in Vienna. "But these negotiations are the best chance we have ever had."
It took months of very hard bargaining for the two sides to agree to try to negotiate a final deal, and Tuesday's talks will be the first test of that resolve, the BBC's Bethany Bell in Vienna reports.
The West wants Iran to scale back permanently its sensitive nuclear activities to ensure that it cannot assemble a nuclear weapon. But Iran says its nuclear work will continue and wants an end to the sanctions that have battered its economy.
The interim deal, which was struck in November but only took effect on 20 January, gives Iran and the P5+1 six months to negotiate a "comprehensive solution", although it can be extended.
It obliges Iran to: halt production of near-20% enriched uranium, which could be further enriched to weapons-grade relatively quickly; dilute half of its stockpile; commit not to commission or fuel the Arak heavy-water reactor, from where spent fuel plutonium could be extracted; and submit to daily inspections by the International Atomic Energy Agency (IAEA).
In return, the P5+1 has eased sanctions on trade in petrochemicals, precious metals and on the provision of insurance for oil shipments. It has also committed to give Iran access to $4.2bn of oil revenue frozen in foreign banks at monthly intervals.
The issues for the permanent deal are said to include the level of Iran's ongoing enrichment of uranium; the underground enrichment facility at Fordo; the Arak reactor; and Tehran's willingness to let inspectors visit the Parchin military complex, where the IAEA suspects activities "relevant to the development of a nuclear explosive device" were carried out.
Iran has a number of "red lines", including not dismantling any facilities.