The deal concluded with major powers - which aims to prevent Iran from developing a nuclear bomb in return for far-reaching sanctions relief - had been at risk ever since Washington pulled out in 2018 and reimposed tough economic restrictions, prompting Tehran to scale back its compliance.
Observers in Tehran believe the government's decision to cease complying with the agreement - rather than just implementing another phase of its partial withdrawal - is a reaction to the US army killing top Iranian general Qassem Soleimani.
Earlier on Sunday, Foreign Ministry spokesman Abbas Mousavi had announced that Iran would take only one further step in the country's partial withdrawal from the international agreement. "We will have an important meeting on this in the evening and decide on the fifth phase of the partial withdrawal," the news agency Isna quoted him as saying.
Iran has sworn "harsh retaliation" for the killing of the general. Fears are spreading of a further escalation of the situation and possible armed conflicts.
In November, the International Atomic Energy Agency (IAEA) confirmed in a report that Iran had far surpassed its permitted stocks of enriched uranium - a fissile material that could be used in warheads if it were processed further.
The enrichment process increases the percentage of the U235 uranium isotope, which is much easier to split apart in a fission reaction. Naturally occurring uranium contains only 0.7 per cent of the U235 isotope; conventional nuclear power reactors require enriched uranium with 3.5 to 4.5 per cent U235.
With the correct know-how and modern centrifuges, uranium can be enriched to 90 per cent U235, a level that allows the construction of an atomic bomb.
While Iran said on Sunday it would no longer put a limit on the number and models of its centrifuges or on storing enriched uranium, it also said that its nuclear programme was geared towards its technological needs - thereby leaving open the level of enrichment.
It remains unclear to what extent Iran wants to enrich its uranium.
Iran also said it would continue to cooperate with the IAEA and would be willing to return to the nuclear deal if it were implemented and the US sanctions lifted.
Observers said this indicates that the Iranian government in Teheran has again left a back door open for a potential diplomatic solution.
Former US national security adviser John Bolton took a hard line, however. "Another good day. Iran rips the mask off the idea it ever fully complied with the nuclear deal, or that it made a strategic decision to forswear nuclear weapons," he said on Twitter.
"Now, it's on to the real job: effectively preventing the ayatollahs from getting such a capability," Bolton added.
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