Iran and world powers agreed Thursday a timetable and framework for the ambitious and arduous process of hammering out a lasting nuclear accord by July 20 that satisfies all sides.
Such a deal, if reached, should resolve the decade-old standoff over Iran's nuclear drive, silence talk of war for good and help normalise Tehran's strained international relations.
After chairing "very productive" days of talks in Vienna, EU foreign policy chief Catherine Ashton said experts would meet in early March before political directors return to Vienna on March 17.
"There is a lot to do, it won't be easy but we have made a good start," Ashton told reporters, saying negotiators had "identified all the issues we need to address".
Iranian Foreign Minister Mohammad Javad Zarif wrote on his Facebook page that the parties "also agreed to hold several meeting on a monthly basis until late May".
He told Iranian media that the atmosphere in the Austrian capital was "very serious" and "even a little bit more positive than anyone predicted," but said there was a "difficult way ahead of us".
A senior US administration official said the discussions were "very workmanlike" and "substantive, covering all the issues that need to be on the table... We are long past speeches of ideology".
Iran and the United States, China, Russia, Britain, France and Germany aim to transform a landmark but only interim deal struck in Geneva in November into a long-term agreement.
Diplomats said they aimed to nail down the final accord by the time a six-month freeze of certain activities agreed in Geneva expires on July 20, although this period can be extended.
The UN atomic agency said in a monthly update Thursday that after one month, Iran was still sticking to its commitments to limit uranium enrichment to low purities and not to install any new equipment.
The holy grail for the long-term accord is for Iran to retain its civilian nuclear programme, but likely on a reduced scale and with enhanced oversight to ensure a dash for nuclear weapons is all but impossible.
This could involve closing the underground Fordo facility, slashing the number of uranium centrifuges, cutting fissile material stockpiles, altering a new reactor being built at Arak and tougher UN inspections.
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In exchange, all UN Security Council, US and EU sanctions on Iran -- which are costing it billions of dollars every week in lost oil revenues, wreaking havoc on its economy -- would be lifted.
-- Iran's 'red lines' -
But whether Iran will play along remains to be seen. It has set out a number of "red lines" which the commander of the hardline Revolutionary Guards warned Wednesday must not be crossed.
Tehran has previously indicated its opposition to dismantling any nuclear facilities, for example, and has long insisted on its "right" to uranium enrichment.
Daryl Kimball from the Arms Control Association called Thursday's framework "an important step forward that makes it more likely the two sides can arrive at a realistic, comprehensive deal in the next 6-12 months."
Western nations and Israel have long suspected Iran of covertly pursuing a nuclear weapons capability alongside its civilian programme, charges denied by Tehran.
Any deal will have to be sold not only to other countries like Israel -- assumed to have nuclear weapons itself -- and the Sunni Gulf monarchies, but also to hardliners in Washington and Tehran.
Chief US negotiator Wendy Sherman will head from Vienna to Israel to brief the government of Prime Minister Benjamin Netanyahu, and to Riyadh, Abu Dhabi and Dubai, the State Department said.
US President Barack Obama has members of Congress breathing down his neck, threatening more sanctions and demanding -- with Israel -- a total dismantlement of Iran's nuclear facilities.
Iranian President Hassan Rouhani, whose election in 2013 has helped thaw relations with the West, is on thin ice with hardliners seeking to turn supreme leader Ayatollah Ali Khamenei against him.