Tehran's announcement came exactly a year after the US withdrew from the deal, known as the Joint Comprehensive Plan of Action (JCPOA), with further measures threatened if the agreement's other signatories fail to mitigate the impact of renewed American sanctions within 60 days.

- What is the significance of Iran's announcement? -

Experts said Iran's announcement has come in reaction to domestic pressure over the crippling impact of US sanctions.

Rather than a serious escalation of its nuclear programme, it is being read as a way of putting pressure on the other signatories -- notably Germany, France and the UK.

Ellie Geranmayeh of the European Council on Foreign Relations told AFP that Tehran's announcement was a "real warning shot", showing a shift from an approach "of strategic patience to strategic action".

Robert Kelley, who previously worked in the US's nuclear weapons programme and is now at the Stockholm International Peace Research Institute (SIPRI), said Iran's announcement was a measure "to save face".

Iran needs neither more enriched uranium stocks nor the heavy water -- as they don't have a reactor that uses heavy water anymore -- while new US sanctions prevented it from selling any excess, he added.

- Is Iran going to try to build a nuclear weapon? -

Kelley told AFP that, despite Wednesday's announcement, there would be "an infinite way" to go for Iran to build a weapon as, for one, it requires uranium enriched to a much higher degree, and Iran is under daily inspections by the UN's nuclear watchdog, the IAEA, which monitors the deal's implementation.

"As long as everything is under IAEA safeguards and inspected daily, everyone will know exactly what is going on," said Kelley, a former director of IAEA nuclear inspections.

The IAEA would find out "within a week" when the stocks of enriched uranium and heavy water had exceeded the prescribed limits, he said.

- How will other signatories react? -

In their first reactions, Britain, Germany and France have already condemned Iran's announcement, urging it to uphold the nuclear pact.

Geranmayeh said it was unlikely deal signatories would re-instate sanctions on Iran at this point.

"There is nothing to be gained at the moment by the Europeans joining the maximum pressure campaign of the United States other than potential military conflict on their doorstep," she said.

But if they didn't want Iran to break further commitments, deal signatories would have to step up their game, she added. That for example could mean pressuring Washington to lift sanctions and implementing INSTEX, a trade mechanism Britain, France and Germany introduced in January to allow Tehran to keep trading with EU companies bypassing US sanctions.

Iranian Foreign Minister Mohammad Javad Zarif has accused in particular the European signatories of not fulfilling their obligations under the deal, issuing "good statements" but not following through with action.

- Can the deal survive? -

The UK's former ambassador to Tehran, Richard Dalton, told AFP how the deal could continue to exist was not clear.

"It's not yet clear whether the European governments feel they can do anything in the next 60 days that they have been unable to do in the last 365," said Dalton, now president of the British Iranian Chamber of Commerce.

Trita Parsi, founder and former president of the National Iranian American Council and a vocal supporter of the agreement, said Europe was in a "difficult spot".

"If it fulfils its obligations under the nuclear deal, it will face costly tensions with Washington."

But if the EU toes the American line, it would "face the risk of (US national security adviser) John Bolton starting a war with Iran whose repercussions to Europe will be far greater than that of the Iraq war," Parsi said in a statement.

Dalton said the US could yet succeed in making the deal collapse.

"Not only is the United States destroying the EU's economic sovereignty -- and acting directly against EU security interests and global non proliferation -- but they're also seriously risking a war," he said.