Here are key developments in the bitter standoff.

- US quits -

On May 8, 2018, President Donald Trump announces the US withdrawal from the 2015 pact, saying "we cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement".

The move heralds the reinstatement of US sanctions.

The US warns other countries to end trade and investment in Iran and to stop buying its oil or face punitive measures.

But US allies Britain, France, Germany -- who were also parties to the deal alongside Russia and China -- insist Iran has abided by its commitments and say they are determined to save the agreement.

- Twelve conditions -

Iranian President Hassan Rouhani says Tehran would be within its rights to scrap the curbs it agreed in the deal. But he calls on the remaining parties to save it.

On May 21, US Secretary of State Mike Pompeo outlines 12 tough conditions from Washington for any "new deal". They include not only new nuclear commitments but also a complete scaling back of Iran's regional role.

- Sanctions -

On August 7, Washington reimposes a first set of sanctions that target Iran's access to US banknotes and key industries, including cars and aviation.

Major international firms halt their activities or projects in Iran.

On November 5, the United States reimposes a second package of sanctions aimed at significantly reducing Iran's oil exports and cutting it off from international finance.

At the end of January 2019, Britain, France and Germany launch a trade mechanism in a bid to allow Tehran to keep trading with EU companies bypassing US sanctions. No transaction has taken place under the system to date.

- 'Terrorism' -

On April 8, Washington designates Iran's ideological army, the Revolutionary Guards, a "foreign terrorist organisation".

Tehran immediately declares Washington a "state sponsor of terrorism" and blacklists its forces in the region as "terrorist groups".

- End of waivers -

On April 22, Trump announces his decision to cancel sanctions exemptions enjoyed by eight countries on oil imports.

On May 8, Iran says it has decided to suspend commitments it made under the nuclear deal, some immediately and some after 60 days if no progress is made on sanctions relief.

They include restrictions on the level to which Iran can enrich uranium and modifications to Iran's Arak heavy water reactor that were designed to prevent the production of plutonium as a byproduct.

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

- Escalation in the Gulf -

On May 12, four ships, including three oil tankers, are the target of mysterious "sabotage" acts in United Arab Emirates waters. The United States and Saudi Arabia accuse Iran, which denies any involvement.

On June 13, two oil tankers are attacked in the Gulf of Oman. Washington, London and Riyadh accuse Iran, which again denies involvement.

On June 20, Iran's Revolutionary Guards say they shot down a US spy drone which allegedly violated Iranian airspace. The Pentagon denies the unmanned aircraft entered Iran's airspace.

The next day Trump approves a retaliatory strike, but cancels it at the last minute.

- New sanctions -

On June 24, the United States imposes "hard-hitting" financial sanctions on Iran's supreme leader Ayatollah Ali Khamenei and senior Iranian military leaders.

Iran accuses Washington, which says it wants to talk, of closing the path to diplomacy.