Here is a timeline:

- US pullout -

On May 8, 2018, President Donald Trump unilaterally pulls the United States out of the accord and announces the reimposition of sanctions against Iran and companies with ties to it.

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

Iran has always denied its nuclear programme has any military dimension.

Tehran urges the remaining parties -- Britain, China, France, Germany and Russia -- to salvage the deal.

- US sanctions -

On August 7, Washington imposes sanctions targeting access to US banknotes and key industries such as cars and carpets.

New sanctions on November 5 hit Iran's vital oil sector and central bank transactions.

Major international firms halt their activities or projects in Iran.

In May 2019 Washington ends its sanctions exemptions on eight countries buying Iranian crude.

- Iran starts walk-back -

Iran on May 8, 2019 announces its first step back from the deal, saying it will suspend commitments on limiting the amount of heavy water and enriched uranium it possesses.

Trump announces new measures against its steel and mining sectors.

On July 1, Iran says it has exceeded the 300-kilogram limit on its enriched uranium reserves.

Six days later, it confirms it has also breached the accord's uranium enrichment cap of 3.67 percent.

- 'Highest sanctions' -

On September 4, Iranian President Hassan Rouhani lifts all limits on nuclear research and development.

Tensions soar after a wave of aerial attacks on September 14 on two major Saudi oil facilities, blamed on Tehran. It denies involvement.

On September 20, Trump announces new sanctions on Iran's central bank, calling them "the highest sanctions ever imposed on a country".

- Iran takes more steps -

On September 26, the IAEA nuclear watchdog says Iran has started using advanced centrifuges to enrich uranium.

On November 4, Tehran says its enrichment has reached five kilograms per day, more than a tenfold increase, and announces it has developed two new advanced centrifuges.

It comes after the expiry of a Tehran deadline for the remaining parties to the deal to create a mechanism for foreign firms to continuing doing business in Iran.

On November 7, Iran resumes uranium enrichment at its underground Fordo plant -- its fourth walkback from the accord.

On the 18th, it says Iranian heavy water reserves have passed the limit fixed by the accord.

- Concern mounts -

On November 27, France raises the possibility of triggering a dispute mechanism in the deal that could lead to the reimposition of UN sanctions against Iran.

On December 5, Britain, France and Germany accuse Iran, in a letter to the UN, of developing nuclear-capable ballistic missiles.

Iranian Foreign Minister Mohammad Javad Zarif dismisses the charge as "a desperate falsehood".

- 'Fifth phase' -

On January 5, 2020, Iran announces its fifth step back from the deal, saying it will forgo a "limit on the number of centrifuges" while stressing it will continue cooperation with the IAEA.

The announcement comes after a US drone strike killed top Iranian military commander Qasem Soleimani in Iraq, sparking fury in Iran.

"Iran's nuclear programme no longer faces any limitation in the operational field," it says.