Iran has declared it will no longer abide by any of the restrictions imposed by the 2015 nuclear deal.
In a statement it said it would no longer observe limitations on its capacity for enrichment, the level of enrichment, the stock of enriched material, or research and development.
The statement came after a meeting of the Iranian cabinet in Tehran.
Tensions have been high over the killing of Iranian General Qasem Soleimani by the US in Baghdad.
Reports from Baghdad say the US embassy compound there was targeted in an attack on Sunday evening. A source told the BBC that four rounds of "indirect fire " had been launched in the direction of the embassy. There are no reports of casualties.
Hundreds of thousands turned out in Iran on Sunday to give Soleimani a hero's welcome ahead of his funeral on Tuesday.
Under the 2015 accord, Iran agreed to limit its sensitive nuclear activities and allow in international inspectors in return for the lifting of crippling economic sanctions.
US President Donald Trump abandoned it in 2018, saying he wanted to force Iran to negotiate a new deal that would place indefinite curbs on its nuclear programme and also halt its development of ballistic missiles.
Iran refused and had since been gradually rolling back its commitments under the agreement.
Earlier on Sunday, Iraqi MPs passed a non-binding resolution calling for foreign troops to leave the country after the killing of Soleimani in a drone strike at Baghdad airport on Friday.
About 5,000 US soldiers are in Iraq as part of the international coalition against the Islamic State (IS) group. The coalition paused operations against IS in Iraq just before Sunday's vote.
Mr Trump has again threatened Iran that the US will strike back in the event of retaliation for Soleimani's death, this time saying it could do so "perhaps in a disproportionate manner".
The 2015 nuclear agreement with Iran, on life support ever since the Trump administration abandoned it in May 2018, may now be in its final death throes.
Donald Trump, throughout his presidential campaign and then as president, has never failed to rail against what he calls his predecessor President Barack Obama's "bad deal". But all of its other signatories – the UK, France, Russia, China, Germany and the EU – believe that it still has merit.
The agreement, known as the JCPOA, constrained Iran's nuclear programme for a set period in a largely verifiable way but its greatest significance – even more so given the current crisis – is that it helped to avert an imminent war. Before its signature there was mounting concern about Tehran's nuclear activities and every chance that Israel (or possibly Israel and the US in tandem) might attack Iran's nuclear facilities.
Since the US withdrawal, Iran has successively been breaching some of the key constraints of the JCPOA. Now it appears to be throwing these constraints over altogether. What matters now is precisely what it decides to do. Will it up its level of uranium enrichment, for example, to 20%? This would reduce significantly the time it would take Tehran to obtain suitable material for a bomb. Will it continue to abide by enhanced international inspection measures?
We are now at the destination the Trump administration clearly hoped for in May 2018 but the major powers, while deeply unhappy about Iran's breaches of the deal, are also shocked at the controversial decision by Mr Trump to kill the head of Iran's Quds Force, a decision that has again brought the US and Iran to the brink of war.
What did Iran say?
Iran had been expected to announce its latest stance on the nuclear agreement this weekend, before news of Soleimani's death.
A statement broadcast on state TV said the country would no longer respect any limits laid down in the 2015 deal.
"Iran will continue its nuclear enrichment with no limitations and based on its technical needs," the statement said.
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