European powers have triggered a formal dispute mechanism over Iran's rolling back of key commitments under the 2015 nuclear deal, pushing it closer to total collapse.
Since July, Iran has gradually lifted all limits on its production of enriched uranium, which it has said it is entitled to do in response to sanctions the US reinstated when it abandoned the accord in 2018.
France, Germany and the UK said they did not accept Iran's argument and had started the dispute process with the aim of saving the deal through dialogue.
Under the nuclear deal, Iran agreed to limit its sensitive activities and allow in international inspectors in return for the lifting of international sanctions.
What is enriched uranium?
Enriched uranium is produced by feeding uranium hexafluoride gas into centrifuges to separate out the most suitable isotope for nuclear fission, called U-235.
Low-enriched uranium, which typically has a 3-5% concentration of U-235, can be used to produce fuel for commercial nuclear power plants.
Highly enriched uranium has a concentration of 20% or more and is used in research reactors. Weapons-grade uranium is 90% enriched or more.
Under the nuclear deal, Iran is allowed to enrich uranium only up to a 3.67% concentration; to stockpile no more than 300kg (660lbs) of the material; to operate no more than 5,060 of its oldest and least efficient centrifuges; and to cease enrichment at the underground Fordo facility.
Another part of the deal instructs Iran not to accumulate more than 130 tonnes of heavy water, which contains more hydrogen than ordinary water, and to redesign its heavy-water nuclear reactor at Arak. Spent fuel from a heavy-water reactor contains plutonium, which can be used in a nuclear bomb.
What has Iran done?
In response to what it considers as the failure of other parties to abide by the nuclear deal, Iran has taken five steps to "reduce" its commitments:
- On 1 July 2019, it lifted the limits on its stockpiles of enriched uranium and heavy water
- On 7 July, it began enriching uranium to 4.5% concentration so it could provide fuel for the Bushehr power plant - beyond the 3.67% cap
- On 6 September, it lifted "all limits" on research and development of centrifuge technology and began to install more advanced centrifuges
- On 5 November, it resumed enrichment at Fordo
- On 5 January 2020, it lifted the limit on the number of centrifuges in operation
Iran said the fifth step meant there were no longer any restrictions on its enrichment programme and that operations would "proceed based on its technical requirements from now on".
But it added that it would continue to co-operate with the global watchdog that monitors the nuclear deal, the International Atomic Energy Agency (IAEA), and that it was ready to reverse the five steps if the US sanctions were lifted.
Why do Iran's actions matter?
The Arms Control Association, a US-based advocacy group, said in December 2019 that the steps Iran had taken until then appeared to be designed to increase pressure on European powers to deliver on sanctions relief and were "not indicative of a dash to a nuclear bomb".
"While concerning, the breaches do not pose a near-term risk and are quickly reversible, supporting [President Hassan] Rouhani's assertion that Iran will return to compliance with the [nuclear deal] if its conditions are met," it added.
The ACA noted that the breach in July of the 300kg enriched uranium stockpile limit only marginally shortened Iran's "break-out time" - the time it would theoretically take to acquire enough fissile material for one bomb.
To manufacture one bomb, Iran would need to produce 1,050kg of 3.67% enriched uranium and would then need to further enrich that to 90% or more, the ACA said. In November, the IAEA said Iran had 372kg of low-enriched uranium.
Iran's decision to increase the level of uranium enrichment could also pose a long-term proliferation risk, according to the ACA.
That is because going from uranium's natural state of 0.7% concentration of U-235 to 20% takes about 90% of the total effort required to get to weapons-grade.
Before the nuclear deal, Iran had a sufficient amount of 20% enriched uranium and number of centrifuges that its "break-out time" was estimated to be about two to three months.
The deal slowed the "break-out time" to at least a year. But the reversal of Iran's commitments on enrichment could speed that up.
The use of advanced centrifuges, which can enrich uranium faster and more efficiently, would allow Iran to accumulate enriched uranium more quickly.
The resumption of enrichment at Fordo is troubling because the facility is built beneath a mountain and is relatively protected from a military strike.
Why did Iran stop abiding by its commitments?
The Iranian economy has slumped since President Donald Trump withdrew from the deal in May 2018 and began reinstating sanctions. He said the deal was flawed and that he wanted to force Iran's government to renegotiate the terms - something it refused to do.
The other parties to the deal - the UK, France, Germany, China and Russia - criticised Mr Trump's decision and said they remained committed to the deal.
In May 2019, the White House stepped up pressure on Iran by ending exemptions from secondary sanctions for countries still buying Iranian oil.
It also ended exemptions for countries participating in deals under which Iran exchanged its surplus low-enriched uranium for un-enriched ore concentrate known as "yellowcake" and sold its surplus heavy water. Such transfers allowed Iran to continue production of both materials without exceeding the stockpile limits.
Iran's President Rouhani subsequently said it would retaliate against the US sanctions by suspending its commitment to comply with the stockpile caps.
Officials noted that article 36 of the nuclear deal allowed one party to "cease performing its commitments… in whole or in part" in the event of "significant non-performance" by other parties. They said the US had violated the deal the previous year and that European countries had failed to deliver its promised benefits.
The EU set up a mechanism for facilitating trade, known as Instex, which essentially allowed goods to be bartered between Iranian and foreign companies without direct financial transactions. It became operational in June 2019, but Iran said it did not meet its needs.
Following Iran's decision to lift the last limit on uranium enrichment in January 2020, France, Germany and the UK said they had "no choice" but to trigger the deal's Dispute Resolution Mechanism.
"Iran's actions are inconsistent with the provisions of the nuclear agreement and have increasingly severe and non-reversible proliferation implications," they said.
"We do this in good faith with the overarching objective of preserving the [nuclear deal] and in the sincere hope of finding a way forward to resolve the impasse through constructive diplomatic dialogue, while preserving the agreement and remaining within its framework," they added.
Iran warned the Europeans that if they abused the mechanism they would face "consequences", but stressed it would also "welcome any practical initiatives".
If the steps Iran has taken are judged to constitute "significant non-performance", the other parties can ask the UN Security Council to "snap back" the sanctions that were lifted. No member of the Security Council could veto such a move.
Does Iran want a nuclear bomb?
Iran insists it has never sought to develop such a weapon.
The international community does not believe that, pointing to evidence collected by the IAEA suggesting that until 2003 Iran conducted "a range of activities relevant to the development of a nuclear explosive device". Some of those activities continued until 2009, according to the IAEA.
In 2018, Israel displayed what it said were archives it secretly took from Iran which showed Iran continued to pursue nuclear weapons knowledge after 2015 - though Iran called the accusation "ridiculous".
The US intelligence community nevertheless assessed in January 2019 that Iran was "not currently undertaking the key nuclear weapons-development activities we judge necessary to produce a nuclear device".
In November 2019, the IAEA called on Iran to explain why it had "detected natural uranium particles of anthropogenic [human induced] origin at a location in Iran not declared to the agency".
The IAEA did not name the location. But inspectors were believed to have taken samples from a location in Tehran's Turquzabad district, where Israeli Prime Minister Benjamin Netanyahu said there was a "secret atomic warehouse".