Geneva (AFP) - US Secretary of State John Kerry and his Iranian counterpart are due to meet in Switzerland on Sunday, seeking to agree the outlines of a nuclear deal by a March 31 deadline.
Few insights into the comprehensive accord under discussion since November 2013 have leaked, with officials remaining tight-lipped to protect the high stakes in the negotiations.
But US officials have laid out some of what they called the "bottom lines" of any deal, though without going into specifics.
Here are some of the possible contours of a deal:
To reach a verifiable comprehensive agreement that limits Iran's ability to harness enough fissile material to build a nuclear bomb. In return, the international community would initially ease and then lift all sanctions imposed on the Islamic republic.
The negotiations have been extended twice but all sides say this will be the last round, having set a March 31 deadline for a political deal.
A final accord has to follow by the end of June.
World powers want to cut Iran's ability to build an atomic weapon to a "one-year breakout time." That would mean Tehran would need at least 12 months to be able to produce enough material for a nuclear bomb.
The assumption is that the international community would have enough time to detect such a move -- and could seek to strike or destroy the facilities.
This year-long breakout time would stay in place for the length of the deal, which US officials have said they want to be in "double-digits".
Many assume that this figure is pointing to a deal lasting about 10 years, but US officials have refused to comment. France believes 10 years is not enough.
This is one of the trickiest issues. Several years ago, the international community wanted to deny Iran any capability to enrich uranium.
In April 2006, Iran launched a process to enrich uranium to 3.5 percent. By February 2010, Iran had the ability to enrich to 20 percent, giving it the possibility of moving quickly to 90 percent -- the level needed for a bomb.
Iran currently has about 19,000 centrifuges. About 10,200 centrifuges are in operation, used for spinning uranium gas at supersonic speeds to make it suitable for power generation and medical uses but also, at high purities, for a bomb.
According to documents leaked by the Israelis, and deemed accurate by non-proliferation experts, the US wants Tehran to reduce its total number of centrifuges to between 6,500-7,000.
Under the 2013 interim deal, Iran has halted production of 20 percent highly enriched uranium and eliminated or diluted much of its stock down to just five percent in return for limited sanctions relief.
Negotiators now seek to enshrine that agreement and cut Iran's stockpile of low-enriched uranium gas. A senior US official said Tehran would have to "significantly reduce" both the number of centrifuges and its uranium stocks.
There may also be a proposal allowing Iran to ship its uranium gas to Russia, which would convert it to fuel rods for the Bushehr nuclear plant which Moscow helped build.
Research and development:
Certain Western negotiators say that the limits on highly enriched uranium mean nothing if the agreement does not take into account the technological progress made by Iran.
Iran's nuclear chief Ali Akbar Salehi has said the deal would not stop Iran "continuing with force" the development of more powerful and modern centrifuges.
Any deal will have to lay out what nuclear sites Iran would be allowed to maintain. The US does not want Iran to be allowed to develop weapons-grade plutonium at its unfinished Arak reactor.
Plutonium can be used as an alternative fissile material to highly-enriched uranium.
Iran should also not use its Fordo nuclear plant to enrich uranium, which would leave only its Natanz plant capable of enriching uranium.
A tough inspection programme using UN watchdog the IAEA is a cornerstone of any deal to ally fears that Iran could covertly develop a nuclear arsenal. The US bottom line is that Iran must agree to unprecedented inspections of both nuclear and production facilities as well as uranium mines and mills and suspect sites.
Iran wants all sanctions imposed by the US, European Union and United Nations to be lifted. But world powers have refused, talking instead about a phased, gradual easing of the measures. Experts say untangling the sanctions -- from those also imposed for Tehran's alleged terror activities for example -- could in fact be proving one of the most difficult tasks.