The administration of President Donald Trump accused Tehran of "destablising activities" across the region and vowed to exert "maximum pressure" through renewed sanctions to bring them to an end.

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, in two phases.

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.

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

- Support for Iran -

Washington warns on May 21 that Tehran will be hit with the "strongest sanctions in history" unless it concedes to further demands over its missile programme and ends its "destabilising activities".

A top US official says on July 2 that Washington is determined to force Iran to change its policies by slashing its oil export revenues.

On July 6, Tehran's five remaining partners in the accord vow to back "the continuation of Iran's exports of oil and gas".

- War of words -

On July 22, Rouhani warns the US that any conflict with Iran would be the "mother of all wars".

Trump tweets that he should stop making threats "OR YOU WILL SUFFER CONSEQUENCES".

- Sanctions, again -

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

Within hours, German carmaker Daimler says it is halting its activities in Iran. French energy giant Total and other major firms follow suit.

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 known as INSTEX in a bid to allow Tehran to keep trading with EU companies bypassing US sanctions.

- 'Terrorism' -

On March 7, Washington calls for international measures against Iran over three launches it says are related to its missile programme.

On April 2, London, Paris and Berlin call for a UN report on Iran's missile activity.

On April 8, the United States designates Iran's parallel military, the Revolutionary Guard, a "foreign terrorist organisation".

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

- US military deployment -

On May 5, US national security adviser John Bolton says Washington is sending an aircraft carrier strike group and a bomber task force to the Middle East in a "clear and unmistakable" message to Iran.

Two days later, the Pentagon deploys B-52 bombers to the Gulf.

- Iran ripostes -

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