Iran says it has begun work on increasing its uranium enrichment capacity, in case its 2015 nuclear agreement with world powers collapses.
The head of Iran's atomic agency told reporters that it was developing infrastructure to build advanced centrifuges at the Natanz facility.
The agency has informed the United Nations of the move, but said it would remain within the rules of the deal.
President Donald Trump withdrew the US from the deal with Iran last month.
European powers are now scrambling to salvage the nuclear agreement, which imposes restrictions on Iran's nuclear activities in exchange for lifting sanctions.
A spokesman for the International Atomic Energy Agency (IAEA) said on Tuesday that the agency had received a letter from Iran on 4 June informing it that there was a "tentative schedule to start production of UF6", referring to uranium hexafluoride, the feedstock for centrifuges.
What is the work designed to achieve?
The head of Iran's Atomic Energy Organisation, Ali Akbar Salehi, told reporters on Tuesday that preparations were under way to build new centrifuges.
"If we were progressing normally, it would have taken six or seven years, but this will now be ready in the coming weeks and months," he said.
Mr Salehi said this was in line with instructions from Supreme Leader Ayatollah Khamenei, who has ordered officials to be prepared to step up enrichment if the nuclear deal - known as JCPOA - falls apart completely.
"If the JCPOA collapses - please pay attention, if the JCPOA collapses - and if we decide to assemble new centrifuges, we will assemble new-generation of centrifuges. However, for the time being, we move within the framework of the JCPOA," Mr Salehi said.
Is such work allowed under the accord?
Mr Salehi insists Iran is acting "within the framework of the rules and commitments of the nuclear deal".
The accord signed with the US, France, Germany, the UK, Russia, and China, limits uranium enrichment by Iran to 3.67%, far below the roughly 90% threshold of weapons-grade material.
In exchange, the country received relief from crippling sanctions.
Under the agreement, Iran can build parts for the centrifuges as long as it does not put them into operation within the first decade.
President Trump argued that these conditions did not go far enough to curb Iran's nuclear ambitions and pulled out of the agreement, leaving the remaining European signatories scrambling to save it.
Iran insists its nuclear programme is entirely peaceful. Its compliance with the deal has been verified by the IAEA.
What is the purpose of the Natanz facility?
It is Iran's largest uranium enrichment facility, and began operating in 2007 in contravention of UN Security Council resolutions.
It consists of underground buildings capable of holding up to 50,000 centrifuges. Uranium hexafluoride gas is fed into centrifuges, which separate out the most fissile uranium isotope U-235.
The facility produces low-enriched uranium, which has a 3%-4% concentration of U-235.
That can be used to produce fuel for nuclear power plants, but also be enriched to the much higher level of 90% needed to produce nuclear weapons.
A clear signal from Tehran
Analysis by Jonathan Marcus, BBC diplomatic correspondent
This is a clear signal from Tehran that it is not simply a bystander and that if the nuclear deal collapses it has options too.
It comes as key European countries struggle to keep the nuclear agreement on life support.
Major international companies are already beginning to distance themselves from Iran in fear of US sanctions.
The move inevitably increases the sense of tension and it probably does those countries eager to maintain the deal few favours.
It highlights the whole issue of Iran's formerly ambitious enrichment programme and again raises the question as to exactly what this enrichment programme was ultimately for.