Lausanne (AFP) - US Secretary of State John Kerry and his Iranian counterpart meet in Switzerland on Thursday for a final round of talks before a March 31 deadline to agree the outlines of an elusive nuclear deal.
This "framework" accord -- no one knows how detailed it will be -- is meant to be fleshed out into a comprehensive agreement by July 1 that severely restricts Iran's ability to obtain nuclear weapons.
Here are the possible contours of such an agreement, which Iran and the US, China, Russia, Britain, France and Germany (the P5+1) have been negotiating since late 2013.
- UN inspections -
Few details have been leaked, but it appears this final deal would see Iran reduce its nuclear activities in scale and place those that remain under ultra-tight UN inspections.
This would extend the "breakout time" that Iran in theory needs to make one bomb's worth of material -- highly "enriched" uranium or plutonium -- to at least a year from a few months at present.
The assumption is that the international community would then have enough time to detect such a move, through intense diplomatic pressure or military action.
- Centrifuges -
The P5+1 want Iran to slash the number of centrifuges -- enrichment machines -- to several thousand from the current 19,000, around half of which are in operation.
In addition, Iran would close or change the purpose of its virtually impregnable Fordo enrichment facility and send abroad some or all of the uranium already enriched.
The design of a new reactor being built at Arak would meanwhile be changed so it that far less plutonium could be extracted from its spent fuel.
Moreover Iran would limit its research and development of centrifuges able to process uranium several times faster than its current machines.
- Peaceful -
Iran denies seeking nuclear weapons and that it is enriching uranium for peaceful purposes such as nuclear power generation and medicines for cancer patients.
It says it needs to expand greatly its enrichment capacities in order to provide fuel for a future fleet of nuclear power stations.
But the P5+1 say that Iran only has one nuclear power station now, which Russia is under contract to supply until at least 2021, and that others are years if not decades away from being built.
Sending Iran's uranium stocks abroad would allow them to be turned into fuel -- a highly complex process -- by another country, most likely Russia.
- Sanctions -
If Iran agrees to dismantle nuclear facilities, in return it wants the spider's web of sanctions suffocating its economy dismantled too -- and quickly, if not immediately.
But the P5+1 is only prepared to consider suspending -- and not terminating -- sanctions, allowing them to "snap back" if Iran violates the deal.
In addition, they want to stagger the suspensions and link them to certain "milestones" by Iran over the lifetime of the deal, which is likely to be well over a decade.
Moreover, this would only apply to nuclear-related sanctions, and at least initially only unilateral US and EU sanctions imposed since 2010 that target Iran's oil exports and its banks.
Whether this would apply to UN sanctions imposed since 2006 is unclear, with France strictly opposed to any suspension but the United States seen as more amenable.
- Watchdog -
These UN sanctions are based on the UN atomic watchdog, the International Atomic Energy Agency, being unable to vouchsafe for the exclusively peaceful nature of Iran's facilities.
Getting a clean bill of health from the IAEA depends in large part on movement in its stalled probe into allegations, mostly before 2003, that Iran's programme had "possible military dimensions".
The IAEA would also play a vital role in another key element of the mooted deal -- beefed-up inspections to be certain that Iran has no secret nuclear sites for a clandestine effort to make a bomb.