Here is a timeline:

- US withdrawal -

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 US -- plus Germany on May 8, 2018.

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

- US sanctions -

Later that year, 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 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 circumvent 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 2019, the International Atomic Energy Agency (IAEA) watchdog says Iran has started using advanced centrifuges to enrich uranium.

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

It then resumes enrichment at its Fordo 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 in January, foregoing a limit on its number of centrifuges.

- Iranian demands -

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

In March, European signatories say they have delivered medical goods to Iran, in the first transaction under a mechanism established to bypass the US sanctions.

On November 27, one of Iran's top nuclear scientists Mohsen Fakhrizadeh is killed near Tehran in an attack on his convoy that Iran blames on Israel.

- New breach -

With Trump in his last days in the White House and tensions between the two countries spiralling, Tehran deals the accord a further blow on January 4, 2021 by saying it has started enriching uranium at Fordo.

In a fresh breach of the deal's limits, the IAEA in February says Iran has started producing uranium metal, which can be used as a component in nuclear weapons.

Iran and the IAEA on February 21 announce a "temporary solution" to allow Iranian facility inspections, giving breathing space for diplomatic negotiation.

Two days later, the IAEA says it is "deeply concerned" by the possible presence of nuclear material at an undeclared site in Iran, as Iran begins to restrict some site inspections by the IAEA in response to the US refusal to lift sanctions.

- Enrichment up to 60 percent -

On April 7, talks on rescuing the accord start in Vienna, the United States invited indirectly for the first time since the arrival at the White House of Joe Biden, who has said he is ready to reintegrate the accord.

Tehran announces on April 10 it has started up or is testing hundreds of advanced centrifuges at its Natanz uranium enrichment plant.

On April 12 it accuses Israel on being behind an attack on its Natanz plant.

The next day it says it will start enriching uranium up to 60 percent, state media reported.