Iran has declared that 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.
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".
These Media Posts will serve as notification to the United States Congress that should Iran strike any U.S. person or target, the United States will quickly & fully strike back, & perhaps in a disproportionate manner. Such legal notice is not required, but is given nevertheless!— Donald J. Trump (@realDonaldTrump) January 5, 2020
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.
Enriched uranium can be used in nuclear weapons.
The statement did not, however, say that Iran was withdrawing from the agreement and it added that Iran would continue to co-operate with the UN's nuclear watchdog, the IAEA.
The statement added that Iran was ready to return to its commitments once it enjoyed the benefits of the agreement.
Correspondents say this is a reference to its inability to sell oil and have access to its income under US sanctions.
Iran has always insisted that its nuclear programme is entirely peaceful.
Sanctions have caused Iran's oil exports to collapse and the value of its currency to plummet, and sent its inflation rate soaring.
How has the international community reacted?
The other parties to the 2015 deal - the UK, France, Germany, China and Russia - tried to keep the agreement alive after the US withdrew in 2018.
EU foreign policy chief Josep Borrell has invited Iran's Foreign Minister, Mohammad Javad Zarif, to visit Brussels to discuss both the nuclear deal and how to defuse the crisis over the Soleimani assassination.
German Chancellor Angela Merkel has agreed with French President Emmanuel Macron and UK Prime Minister Boris Johnson to work towards de-escalation in the Middle East, a German government spokesman was quoted as saying by AFP news agency.
Johnson said "we will not lament" the death of Soleimani, describing him as "a threat to all our interests".