Iran is to start injecting uranium gas into more than 1,044 centrifuges as it moves away from a landmark nuclear deal agreed in 2015.
The country's president, Hassan Rouhani, said it would take place at its underground Fordow centre as "part of our fourth step to reduce our nuclear commitments to the deal."
Uranium gas injection can allow enriched uranium to be produced - currently banned at the site under the pact.
Highly enriched uranium can be used to make nuclear weapons.
President Donald Trump pulled the US out of the Obama-brokered deal in May 2018 and reimposed tough sanctions on Iran's economy.
Mr Rouhani - speaking live on national TV - gave Britain, France and Germany another two months to try to salvage the agreement by protecting Iran from the sanctions.
"We can't unilaterally accept that we completely fulfil our commitments [to the deal] and they don't follow up on their commitments," Mr Rouhani said in his TV statement.
He added: "All these measures are reversible if other parties fulfil their commitments... We should be able to sell our oil and to transfer its money into the country."
