The hard-won accord started unravelling 18 months ago when the United States pulled out, having been instrumental in the 21 months of negotiations it took to conclude the deal between Iran and major powers.

Iran has said all its steps are transparent and reversible if the remaining parties to the agreement find a way to bypass US sanctions.

Here is a timeline:

- US quits -

On May 8, 2018, President Donald Trump announces the United States will quit the 2015 pact and reinstate 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.

Iranian President Hassan Rouhani says he is ready to discuss with the remaining parties -- Britain, China, France, Germany and Russia -- ways to save the deal.

- US sanctions -

In late May, US Secretary of State Mike Pompeo outlines 12 tough conditions from Washington for any "new deal". They include new nuclear commitments and a full scaling-back of Iran's regional role.

In August and November, Washington reimposes sanctions, particularly targeting Iran's oil and finance sectors.

Major international firms halt their activities or projects in Iran.

Trump in May 2019 annuls sanctions exemptions enjoyed by eight countries on Iranian oil imports.

- Iran starts walkback -

A year after the US pullout, 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 Iran's steel and mining sectors.

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

On the 7, it confirms it has also breached the accord's uranium enrichment cap of 3.67 percent.

It threatens to abandon more commitments after 60 days.

A day later, it says it has enriched uranium to 4.5 percent.

- 'Highest sanctions' -

Trump says on August 26 he would be prepared to meet Rouhani "if the circumstances were correct".

The Iranian leader tells Washington to "take the first step" by lifting all sanctions.

On September 4, Rouhani plays down efforts for a diplomatic solution and lifts all limits on nuclear research and development in a new scaling-down of Iran's 2015 commitments.

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 kilogrammes per day, more than a tenfold increase, and announces it has developed two new advanced centrifuges.

That comes after the expiry of a deadline Tehran had set for the remaining parties to the nuclear deal to create a mechanism for foreign firms do business with it to bypass US penalties.

The following day, Rouhani announces the resumption of uranium enrichment at the underground Fordow plant -- its fourth walkback from the accord -- at midnight November 6.

There is a chorus of concern from the remaining parties to the agreement.

On November 7, Iran says it has cancelled the accreditation of an IAEA inspector, who had already left the country.