A 2015 nuclear deal between Iran and world powers is being eroded and efforts to revive the pact face a new challenge with the killing of Tehran's top nuclear scientist.
The accord's restrictions on Iran's atomic work had one objective: to extend the "breakout time" for Tehran to produce enough fissile material for a bomb, if it decided to make one, to at least a year from about two to three months.
Iran maintains that it has never sought nuclear weapons and never would. It says its nuclear work only has civilian aims.
Tehran began breaching the deal's curbs last year in a step-by-step response to President Donald Trump's withdrawal from the deal in May 2018 and the reimposition of U.S. sanctions.
This has shortened the breakout time but reports by the U.N. International Atomic Energy Agency (IAEA), which polices the deal, indicate that Iran is not moving ahead with its nuclear work as fast as it could.
Pictures of the week: November 29 - December 5
Pictures of the week: November 29 - December 5
A woman wearing a face mask takes a photo on her phone in Covent Garden, London, Thursday, Dec. 3, 2020. Britain became the first country in the world to authorize a rigorously tested COVID-19 vaccine Wednesday and could be dispensing shots within days — a historic step toward eventually ending the outbreak that has killed more than 1.4 million people around the globe. (AP Photo/Alberto Pezzali)
A member of staff polishes the 1969 Mercury Cougar XR7 convertible, which had a starring role in the James Bond film 'On Her Majesty's Secret Service', displayed at Bonhams, in Mayfair, before it is offered for the first time at auction, for an estimate price of 100.000 - 150.000 pounds sterling, in London, Thursday, Dec. 3, 2020. (AP Photo/Alberto Pezzali)
A train runs along the tracks in snowy Garsdale, on the border of Cumbria and North Yorkshire with the UK expecting more wintry weather ahead of the first weekend of December, with warnings in place for ice and snow.
Soccer fans wait for the start of the Europa League group B soccer match between Arsenal and Rapid Wien at Emirates stadium in London, England, Thursday, Dec. 3, 2020. (AP Photo/Frank Augstein)
LONDON, ENGLAND - DECEMBER 03: Final decorations are addd to the 40ft Christmas tree ahead of todays re-opening at Somerset House on December 03, 2020 in London, England. (Photo by Tim P. Whitby/Getty Images)
LONDON, ENGLAND - DECEMBER 03: Pop-up winter domes are installed in the courtyard in preparation for a new dining experience, open to the public from December 3rd to January 1st, ahead of todays re-opening at Somerset House on December 03, 2020 in London, England. (Photo by Tim P. Whitby/Getty Images)
Face masks to curb the spread of coronavirus are displayed for sale on a stall as non-essential shops are allowed to reopen after England's second lockdown ended at midnight, on Oxford Street, in London, Wednesday, Dec. 2, 2020. Pfizer and BioNTech say they've won permission Wednesday for emergency use of their COVID-19 vaccine in Britain, the world's first coronavirus shot that's backed by rigorous science -- and a major step toward eventually ending the pandemic. (AP Photo/Matt Dunham)
Elves talk to children in a winter-themed environment at Lapland UK in Whitmoor Forest, near Ascot in Berkshire, which has reopened after England emerged from lockdown.
YORK, ENGLAND - DECEMBER 02: Audience members react as they watch actors from York Theatre Royal perform Jack and the Beanstalk on December 02, 2020 in York, England. The performance is the first of a series that are part of York Theatre Royal's Travelling Pantomime production. Until December 23 the pantomime will visit every one of York's 21 wards and put on a show in community centres, schools, sports and church halls. At the start of each performance the cast will ask the audience to choose between three different panto favourites; Dick Whittington, Jack and the Beanstalk and Snow White and the Seven Dwarfs. Whichever show the audience decide on will be performed. The performance at York Theatre Royal comes on the first day of the new tier system introduced at the end of the second Coronavirus lockdown to prevent against further spread of the Coronavirus. York is in tier two and the restrictions allow for the performance in front of a limited audience to take place. Strict Covid-19 measures will be implemented in line with the current tier two guidelines. (Photo by Ian Forsyth/Getty Images)
Jessica Walker and Nicola Foster, known as the Lido Ladies, swim at Charlton Lido in Hornfair Park, London, on its first day of reopening after the second national lockdown ended and England enters a strengthened tiered system of regional coronavirus restrictions. (Photo by Victoria Jones/PA Images via Getty Images)
Lulu Kingstone, 9, looks up at Neon lights cascading down from one of Kew's heritage trees, during a preview for 'Christmas at Kew' at the Royal Botanic Gardens in Kew, London. (Photo by Andrew Matthews/PA Images via Getty Images)
Barber Ben Fessey cuts the hair of his first customer after reopening his barber shop in Moreton, north west England, on December 2, 2020 as England emerges from a month-long lockdown to combat the novel coronavirus pandemic. - England on December 2 exited a month-long lockdown into a new 3-tiered system of curbs with non-essential retail, leisure centres and salons all reopening but with some sectors, including hospitality, seeing tighter restrictions. (Photo by Paul ELLIS / AFP) (Photo by PAUL ELLIS/AFP via Getty Images)
EDITORIAL USE ONLY Food writer Angela Clutton (centre) and Edd Kimber (second right), the first winner of The Great British Bake Off, with stallholders at the launch of Borough Market's Festive Kitchen, a new three-week livestream, digital pop-up, designed to inspire Londoners and food shoppers across the UK in the run up to Christmas.
A member of staff cleans gym equipment at David Lloyd health club in Leicester as they reopen after the second national lockdown ended and England now has a strengthened tiered system of coronavirus restrictions. (Photo by Joe Giddens/PA Images via Getty Images)
The sun rises over The Solent, which separates the mainland from the Isle of Wight, after the second national lockdown ended and England entered a strengthened tiered system of regional coronavirus restrictions. Only the Isle of Wight, Cornwall and the Isles of Scilly - accounting for little more than 1% of England's population - face the lightest Tier 1 restrictions. (Photo by Steve Parsons/PA Images via Getty Images)
Passengers, wearing masks because of the coronavirus pandemic, exit a London Underground tube train during the morning rush hour in London on December 2, 2020 as England emerges from a month-long lockdown to combat the spread of Covid-19. - England on December 2 exited a month-long lockdown into a new 3-tiered system of curbs with non-essential retail, leisure centres and salons all reopening but with some sectors, including hospitality, seeing tighter restrictions. (Photo by Tolga Akmen / AFP) (Photo by TOLGA AKMEN/AFP via Getty Images)
Student Amy Hariot Ingle, 22, from Newcastle University, performing a Covid-19 test on herself, watched by David Black (right) an occupational heath specialist. Amy will be part of a mass testing across the UK starting tomorrow on December 1 2020.
A man stands by a flock of swans by the Jubilee Pond, Wanstead Flats during the lockdown. (Photo by David Mbiyu / SOPA Images/Sipa USA)
Christmas Lights are lit up in Oxford Circus
Londoners are getting ready to go back to tier 2 when a four-week lockdown in England comes to an end next week. (Photo by Pietro Recchia / SOPA Images/Sipa USA)
The moon passes behind the Shard skyscraper not long after sunrise in London Tuesday Dec. 1, 2020. (AP Photo/Tony Hicks)
EDINBURGH, SCOTLAND - NOVEMBER 30: Edinburgh's Bill Mata in action during the PRO14 match between Edinburgh Rugby and Ulster at BT Murrayfield on November 30, 2020, in Edinburgh, Scotland. (Photo by Ross MacDonald/SNS Group via Getty Images)
No Parking signs seen outside a school in East London. Schools remain open with all precautions being taken to keep pupils safe since they reopened in September as per Government advice. (Photo by David Mbiyu / SOPA Images/Sipa USA)
Britain's Prime Minister Boris Johnson holds a vial of the Oxford/AstraZeneca vaccine Covid-19 candidate vaccine, known as AZD1222, at Wockhardt's pharmaceutical manufacturing facility in Wrexham, Wales, Monday, Nov. 30, 2020. (Paul Ellis/PA via AP)
EU Chief Negotiator Michel Barnier walks with his team to Brexit negotiations at a conference centre, followed by anti-Brexit protester Steve Bray, obscured, holding pro-EU placards in London, Monday, Nov. 30, 2020. Teams from Britain and the European Union are continuing face-to-face talks on a post-Brexit trade deal with little time remaining. (AP Photo/Matt Dunham)
CARDIFF, WALES - NOVEMBER 30: A general view of members of the public walking along the Christmas Market outside Cardiff Castle on November 30, 2020 in Cardiff, Wales. (Photo by Huw Fairclough/Getty Images)
LONDON, UNITED KINGDOM - NOVEMBER 29: An Anatolian shepherd dog, brought by the owner from Turkey, is seen being taken for a walk at the streets of London, on November 29, 2020 in London, United Kingdom. The dog attracted the attention of people in the streets. (Photo by Yunus Dalgic/Anadolu Agency via Getty Images)
Freshley cut trees are loaded onto a trailer at Digby Farm Christmas Trees in Rutland. Families are preparing for Christmas after the UK Government and devolved administrations have agreed a temporary easing of coronavirus restrictions over the festive period, allowing three households to mix in a bubble from December 23 to 27.
A parakeet in a garden in Ealing, London. Photo date: Sunday, November 29, 2020. Photo credit should read: Richard Gray/EMPICS
EU Chief Negotiator Michel Barnier, left, with members of his team as he walks to a conference centre in Westminster in London, Sunday, Nov. 29, 2020. Teams from Britain and the European Union are continuing face-to-face talks on a post-Brexit trade deal in the little remaining time. (AP Photo/Kirsty Wigglesworth)
Players stand and applaud for a minute in memory of Argentine soccer star Diego Maradona prior to the start during the English Premier League soccer match between West Bromwich Albion and Sheffield United at The Hawthorns in West Bromwich, England, Saturday, Nov. 28, 2020. (Jason Cairnduff/Pool via AP)
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European states have sought to save the nuclear deal, pressing Tehran to comply even as Washington has tightened sanctions, and holding out hopes of a change in U.S. policy once President-elect Joe Biden takes office on Jan. 20.
Biden was part of the U.S. administration under Barack Obama that negotiated the 2015 deal.
WHAT HAS IRAN DONE TILL NOW?
Iran has contravened many of the deal's restrictions but is still cooperating with the IAEA and granting inspectors access under one of the most intrusive nuclear verification regimes imposed on any nation.
* Enriched uranium - The deal limits Iran's stockpile of enriched uranium to 202.8 kg, a fraction of the more than eight tonnes it possessed before the deal. The limit was breached last year. The IAEA report in November put the stockpile at 2,442.9 kg.
* Enrichment level - The deal caps the fissile purity to which Iran can refine uranium at 3.67%, far below the 20% achieved before the deal and below the weapons-grade level of 90%. Iran breached the 3.67% cap in July 2019 and the enrichment level has remained steady at up to 4.5% since then.
* Centrifuges - The deal allows Iran to produce enriched uranium using about 5,000 first-generation IR-1 centrifuges at its underground Natanz plant, which was built to house more than 50,000. It can operate small numbers of more advanced models above ground without accumulating enriched uranium. Iran had roughly 19,000 installed centrifuges before the deal.
In 2019, the IAEA said Iran had begun enrichment with advanced centrifuges at an above-ground pilot plant at Natanz. Since then, Iran started moving three cascades, or clusters, of advanced centrifuges to the underground plant. In November, the IAEA said Iran had fed uranium hexafluoride gas feedstock into the first of those underground cascades.
* Fordow - The deal bans enrichment at Fordow, a site Iran secretly built inside a mountain and that was exposed by Western intelligence services in 2009. Centrifuges are allowed there for other purposes, like producing stable isotopes. Iran now has 1,044 IR-1 centrifuges enriching there.
HOW CLOSE IS IRAN TO HAVING A BOMB?
The breaches lengthened the breakout time but estimates still vary. Many diplomats and nuclear experts say the starting point of one year is conservative and Iran would need longer.
David Albright, a former U.N. weapons inspector who tends to have a hawkish position on Iran, estimated in November that Iran's breakout time could be "as short as 3.5 months," although this presumes Iran would use 1,000 advanced centrifuges that were removed under the deal.
WHAT MORE WOULD IRAN NEED TO DO?
If Iran accumulated sufficient fissile material, it would need to assemble a bomb and probably one small enough to be carried by its ballistic missiles. How long that would take exactly is unclear, but stockpiling enough fissile material is widely seen as the biggest hurdle in producing a weapon.
U.S. intelligence agencies and the IAEA believe Iran once had a nuclear weapons program that it halted. There is evidence suggesting Iran obtained a design for a nuclear weapon and carried out various types of work relevant to making one.
Tehran continues to grant the IAEA access to its declared nuclear facilities and allow snap inspections elsewhere.
Iran and the IAEA resolved a standoff this year that had lasted several months over access to two suspected former sites.