Here are key developments in the bitter standoff.

- US quits -

On May 8, 2018, President Donald Trump announces the US will quit the 2015 pact and reinstate sanctions on 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.

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.

But he threatens to resume uranium enrichment if the talks fail.

- 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.

- Progressive disengagement -

On May 8, 2019, Iran says it has decided to suspend two commitments it made under the nuclear deal.

Trump announces new measures against Iran's steel and mining sectors.

On July 1, Iran, which had so far respected its deal commitments according to the UN's nuclear watchdog the IAEA, announces it has exceeded the 300-kilogramme limit on its enriched uranium reserves.

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

It threatens to abandon more commitments after 60 days unless a solution is found.

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

- Diplomatic coup -

On August 25, Iranian Foreign Minister Mohammad Javad Zarif makes a surprise appearance on the sidelines of the G7 summit in Biarritz at the invitation of French President Emmanuel Macron.

Trump says he would be prepared to meet his Iranian counterpart Rouhani "if the circumstances were correct".

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

On September 4, Rouhani tells the cabinet he does not think there will be a deal. Shortly after, the US imposes further sanctions.

- Escalation -

On September 14, a wave of aerial attacks claimed by Yemen's Iran-backed Huthi rebels sparks fires at two major Saudi oil facilities.

Tehran, which denies involvement, is accused by the United States, Saudi Arabia, Britain, France and Germany of being responsible.

The crisis revives fears of a military confrontation between the US and Iran.

It comes after Trump said in June he had approved a retaliatory strike after Iran's Revolutionary Guards shot down a US drone, but cancelled it at the last minute.

- Uranium enrichment -

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

Iran's supreme leader Ayatollah Ali Khamenei on November 3 again rules out negotiations with Washington.

The next day, Tehran says its enriched uranium production has reached five kilogrammes per day, a more than tenfold increase, and it has developed two new advanced centrifuges, one of which is undergoing testing.

Iran will take the fourth step of walking back on the nuclear accord on Tuesday, according to semi-official news agency ISNA.