On Saturday, Washington declared that UN sanctions were back in force, a move other major countries -- including its allies -- said lacked legal basis.

Here is a snapshot:

- US withdrawal -

On May 8, 2018, President Donald Trump withdraws the US from the deal negotiated between Iran and the five permanent members of the UN Security Council -- Britain, China, France, Russia and the United States -- plus Germany.

"We cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement," Trump says.

- US sanctions -

In August and November 2018, Washington reimposes sanctions on Iran and companies with ties to it, notably hitting Iran's vital oil sector and central bank.

Major international firms halt their activities and projects in Iran.

In May 2019, Washington ends its sanctions exemptions for countries buying Iranian crude oil.

- Iran starts walk-back -

Iran that month announces its first step back from the deal, seeking to pressure European signatories to help it get round the sanctions.

Trump then sanctions Iran's steel and mining sectors.

In July, Tehran says it has exceeded the accord's restrictions on its enriched uranium reserves and uranium enrichment level.

- More steps -

In September, the International Atomic Energy Agency (IAEA) nuclear watchdog says Iran has started using advanced centrifuges to enrich uranium.

On November 4, Tehran says its enrichment increased tenfold and that it has developed two new advanced centrifuges.

Later that month, it resumes enrichment at its underground Fordow plant in its fourth walk-back and says its heavy water reserves have passed the accord's limit.

- More centrifuges -

Tensions between Washington and Tehran spiral after a January 2020 US drone strike kills top Iranian commander Qasem Soleimani in Baghdad, sparking a tit-for-tat confrontation.

Iran announces its fifth step back from the deal on January 5, foregoing a limit on its number of centrifuges.

- Dispute mechanism -

On February 14, Iran demands significant economic advantages from Europe in return for cancelling all, or part, of its rollback measures.

On March 31, European signatories say they have delivered medical goods to Iran, in the first transaction under the Instex mechanism set up to bypass the US sanctions.

- Over the limit -

On May 27, Washington ends sanctions waivers for nations that remain in the accord, which allow companies still present in Iran to carry out the agreement.

On June 5, the IAEA says Iran has accumulated enriched uranium at nearly eight times the limit of the accord and has for months blocked inspections at key sites.

On June 19, IAEA governors pass a resolution critical of Iran, the first of its kind since 2012, urging Tehran to provide its inspectors with access to two sites.

- Setback for US -

On August 14, the UN Security Council rejects a US resolution aimed at extending the embargo on arms sales to Iran that expires in October.

On August 20, the United States formally begins the process of activating a controversial mechanism aimed at reimposing UN sanctions on Iran.

It immediately comes up against opposition from European and other powers.

On September 4, the IAEA says Iran has granted its inspectors access to one of the two sites.

Iran's stockpile of enriched uranium now stands at more than ten times the limit set down in the 2015 deal, the IAEA says.

- US reimposes sanctions -

On September 20, Washington unilaterally proclaims UN sanctions against Iran are back in force, and promises to punish those who violate them.

The legal argument for the move is rejected by almost the entire UN Security Council.

The next day, the US names 27 people or entities it says will be subject to UN sanctions.

They include Iran's defence ministry and Atomic Energy Organisation, as well as Venezuelan President Nicolas Maduro, accused of working with the Islamic republic to "flout the UN arms embargo".