But in 2018 US President Donald Trump pulled the United States out of the agreement and reimposed sanctions, prompting Iran to roll back its own commitments.

Despite consistent denials from Tehran, the US and Israel continue to accuse Iran of seeking a nuclear weapon.

Here is a look back at the deal.

- Tough negotiations -

Negotiations start in June 2013 between Iran and the five permanent members of the UN Security Council (Britain, China, France, Russia and the United States) plus Germany.

The final deal is reached in July 2015, seemingly ending a 12-year dispute over the Iran nuclear issue.

- The aim -

The accord's goal is to render it practically impossible for Iran to build an atomic bomb, while allowing it to pursuing a civilian nuclear programme.

Tehran pledges to reduce its nuclear capacities for several years, capping its enrichment of uranium at 3.67 percent -- sufficient for power generation but far below the more than 90 percent required for a nuclear weapon.

Iran agrees to slash the number of its enrichment centrifuges from more than 19,000 to 5,060, for a decade.

It also agrees to modify its heavy water reactor in Arak to prevent it using plutonium for military use.

The deal comes into effect in January 2016.

- Inspectors, sanctions relief -

The International Atomic Energy Agency is charged with inspections to ensure Iran is keeping its side of the bargain.

The accord paves the way for a partial lifting of international sanctions on Tehran, opening the door to foreign investors.

However, UN embargoes on the sale of conventional arms and on ballistic missiles to Iran are maintained up to 2020 and 2023 respectively.

- US exit -

Investment starts to flow into Iran. But in May 2018, Trump pulls out of the agreement, calling it "rotten".

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

From May 2019, Washington ends temporary sanctions waivers it had granted to the main purchasers of Iranian oil.

- Iran rolls back -

Iran in May 2019 announces a first rollback from the accord, saying it will no longer observe restrictions on enriched uranium and heavy water stocks.

It says the European Union, China and Russia must help it circumvent US sanctions and sell oil or it will abandon more terms.

Trump sanctions Iran's steel and iron sectors.

In July, Iran says it has exceeded the deal's uranium enrichment cap.

In September, it lifts restrictions on nuclear research and development and in January 2020 it says it will ignore the limit on its number of centrifuges.

Tensions with Washington spike after a US drone strike kills a top Iranian military commander in Iraq.

- Dispute mechanism launched -

Later in January, the European signatories launch the accord's dispute mechanism, accusing Iran of repeated violations.

In February, Iran says it would be willing to return to the deal if Europe provides "meaningful" economic benefits.

It has insisted all the rollbacks are temporary and easily reversed.

- Nuclear watchdog voices concern -

On March 3, the IAEA says Iran's stockpile of enriched uranium stands at more than five times the limit under the deal. It has also been enriching uranium at a level higher than the 3.67 percent threshold.

The agency's head also demands "clarifications" over an undeclared site in Tehran where uranium particles were found late in 2019.