While Iran's actions will not on their own sound the death knell for the accord, they could mean another stage in its "gradual disintegration" which began with US President Donald Trump's dramatic withdrawal from the agreement last year.

- A painful birth -

The Joint Comprehensive Plan of Action (JCPOA) was finally completed in Vienna on July 14, 2015, after years of arduous negotiations between Iran and the US, France, Britain, Germany, Russia and China.

The JCPOA's core was ensuring that Iran's nuclear programme stayed entirely peaceful. In return, Iran would benefit from the gradual lifting of stifling economic sanctions.

The deal was hailed by its supporters as a big win for multilateralism in general and the Obama administration's foreign policy in particular.

But from early on the deal faced criticism in the US, including from then presidential candidate Trump.

Iran's foes in the region, such as Saudi Arabia and Israel, also insisted it does not provide sufficient protection against the threat of Tehran ultimately acquiring nuclear weapons.

- What the deal entails -

The JCPOA restricts Iran's nuclear programme in several respects and mandates the International Atomic Energy Agency (IAEA) to carry out a strict inspections regime of Iranian facilities.

The restrictions include a limit of 300 kilogrammes on Iran's stockpiles of enriched uranium, which cannot be enriched beyond 3.67 percent.

Its stocks of heavy water are limited to 130 tonnes.

Iran's heavy water reactor in Arak was also subject to modifications rendering it unable to produce substantial quantities of plutonium.

The IAEA says Iran is subject to one of the most stringent inspections regimes anywhere in the world, and that it has access to all the sites necessary to carry out its work.

- Sanctions relief -

The other side of the bargain was to be the lifting of sanctions on Iran, notably in its key oil and gas sectors.

But after Trump's decision to withdraw from the deal in May 2018, the US re-imposed sanctions and sought to force its allies to drastically reduce economic ties with Iran.

The European parties to the JCPOA have tried to respond to the American withdrawal by setting up a special trade mechanism called INSTEX that would allow legitimate trade with Iran to continue without falling foul of US sanctions, but it is still not fully operational.

- Tehran's reaction -

Iran has justified its decision to no longer respect some of the JCPOA's limits by pointing to the fact it is not receiving the economic benefits of the accord following the US withdrawal.

On May 8, Iran said it was going to stop respecting the JCPOA's limits on stockpiles of enriched uranium -- expected to be breached on Thursday -- as well as those for heavy water.

On Tuesday a senior Iranian security official said that as of July 7 it will start abandoning other parts of the deal, namely the 3.67 percent enrichment limit on uranium and curbs on the development of a heavy water reactor.

- What next? -

Reacting to Iran's announcement last week, EU foreign policy chief Federica Mogherini said: "Our assessment on the implementation of the nuclear deal... will never be based on statements but on the evaluation that the IAEA makes."

The IAEA says it can only present the technical facts it garners as a result of its inspections and that ultimately the question of whether the accord is being respected is one for the deal's signatories.

According to Thierry Colville, researcher at Paris's Institute for International and Strategic Affairs (IRIS), the agreement could be in danger of "gradual disintegration" rather than a sudden collapse.

But a more dramatic breakdown of the deal cannot be ruled out if Iran stopped cooperating with the IAEA's inspections, or seriously ramped up enrichment work.

For the moment, "the Europeans, Russia and China remain committed to the nuclear deal and are seeking solutions to address Iran's legitimate frustration with sanctions relief," says Kelsey Davenport from the Arms Control Association.

However, "if the remaining states who are party to the deal abandon that process because of Iran's violations, that will kill the deal," Davenport warned.