Tehran's move comes a year to the day since US President Donald Trump dramatically withdrew from the agreement and proceeded to re-introduce sanctions which have hit the Iranian economy hard.

The UN's Vienna-based nuclear watchdog, the International Atomic Energy Agency (IAEA), has had the delicate task of verifying the deal, known as the Joint Comprehensive Plan of Action (JCPOA), through regular inspections of Iranian facilities.

- Will Iran's move affect inspections? -

Iran did not announce any measures on Wednesday which would appear to impede the IAEA's inspection regime.

"If it's just a matter of now they are starting to accumulate a quantity (of heavy water or uranium) larger than agreed upon, the IAEA will know that within a week," said Robert Kelley, a former director of nuclear inspections at the IAEA.

"As long as everything is under IAEA safeguards and inspected daily, everyone will know exactly what is going on," Kelley added.

The IAEA insists the inspection regime put in place by the JCPOA is the world's toughest, and in each of its quarterly reports on Iran it has confirmed that Tehran is abiding by the deal.

Under the "Additional Protocol" agreed with Iran, inspectors may "conduct complementary access to any location in Iran".

The agency says that its inspection work has doubled since 2013.

IAEA Secretary General Yukiya Amano says the agency's inspectors spend a total of 3,000 calendar days per year on the ground in Iran.

He has also highlighted some 2,000 tamper-proof seals attached to nuclear material and equipment and the "hundreds of thousands of images captured daily by our sophisticated surveillance cameras", the number of which has almost doubled since 2013.

- Why the IAEA? -

Set up in 1957, the IAEA promotes peaceful uses of atomic energy while at the same time overseeing efforts to detect and prevent possible nuclear weapons proliferation.

Because of previous international concern over its nuclear programme, Iran agreed in 2003 to allow snap IAEA inspections of its nuclear facilities.

However, cooperation broke down in 2006. The IAEA referred Iran to the UN Security Council, which went on to impose sanctions, and Iran halted enhanced IAEA inspections.

A renewed diplomatic push eventually led to the JCPOA in 2015, under which the agency is charged with regular inspections of declared facilities in Iran such as uranium mines and centrifuge workshops for up to 25 years.

The aim is to ensure that Iran is not holding undeclared stocks of nuclear material and is not enriching uranium past a certain level.

- Political pressures -

In addition to the US withdrawing from the deal, Israel -- Iran's regional arch-foe -- has also been highly critical of the JCPOA.

In August 2017, Washington's envoy to the UN Nikki Haley urged the IAEA to widen its inspections, including to military sites.

A year later in an address to the UN General Assembly, Israeli Prime Minister Benjamin Netanyahu claimed Iran had a "secret atomic warehouse" as part of a clandestine nuclear programme and called on the IAEA to inspect the site immediately.

In January, Amano rejected pressure on the agency, saying: "If our credibility is thrown into question, and, in particular, if attempts are made to micro-manage or put pressure on the Agency in nuclear verification, that is counter-productive and extremely harmful."

In its most recent reports on the JCPOA, the agency has also taken to reminding Iran that "timely and proactive co-operation" in providing access to locations it wishes to inspect would "enhance confidence".