Iran has moved to stop UN inspections of its nuclear sites and step up uranium enrichment under a new law approved by its parliament.
The bill would require the government to resume enriching uranium to 20 per cent - well above the 3.67 per cent agreed under a 2015 nuclear deal - if crippling sanctions are not eased in two months.
Iran's President Hassan Rouhani said he opposed the implementation of the law, reports the BBC.
It comes after the targeted killing of Iran's top nuclear scientist.
Mohsen Fakhrizadeh was killed in a mysterious attack on a road outside the capital Tehran last Friday. Iran believes Israel and an exiled opposition group used a remote-control weapon to carry out the shooting.
Israel has not publicly commented on the allegations of its involvement.
Fakhrizadeh played a crucial role in Iran's nuclear programme, but the government insists its nuclear activities are entirely peaceful.
It has been subjected to crippling Western sanctions aimed at preventing it from developing nuclear weapons.
What does Iran's new law mean for its nuclear programme?
Under the law, ratified by Iran's Guardian Council, Tehran would give two months for the European signatories of the 2015 nuclear agreement to work to ease sanctions on Iran's oil and financial sectors imposed when the US abandoned the deal in 2018.
If the sanctions had not been eased by the deadline, the government would then increase uranium enrichment to 20 per cent and install advanced centrifuges, which are used to enrich uranium, at its nuclear facilities at Natanz and Fordow.
It would also block UN inspectors from accessing these sites.
"Today in a letter, the parliament speaker officially asked the president to implement the new law," Iran's Fars news agency reported on Wednesday.
Before the law was ratified, President Rouhani said his government did not agree with the legislation, which he described as "damaging for diplomacy".
US President Donald Trump pulled out of the 2015 nuclear agreement in May 2018, and reimposed strict economic sanctions against Tehran.
President-elect Joe Biden has said he would return the US to the agreement - negotiated under Barack Obama - and would lift sanctions if Tehran returned to "strict compliance with the nuclear deal".
Mr Biden, who is due to be sworn in as the 46th US president on 20 January, told the New York Times that "it's going to be hard", but that "the last goddamn thing we need in that part of the world is a build-up of nuclear capability".
Iran breached the 3.67 per cent cap in July 2019 and the enrichment level has remained steady at up to 4.5 per cent since then.
Low-enriched uranium - which typically has a 3-5 per cent concentration of uranium-235 - can be used to produce fuel for power plants. Weapons-grade uranium is 90 per cent enriched or more.
Suspicions that Iran was using its nuclear programme as a cover to develop a nuclear bomb prompted the EU, US and UN to impose sanctions in 2010.
The 2015 deal was designed to constrain the programme in a verifiable way in return for sanctions relief.