According to an AEOI statement, the operation started at 12 a.m. on Thursday after the transfer of a 2,800-kg cylinder containing 2,000 kg of UF6 (uranium hexafluoride) from Natanz nuclear facility to Fordow - near the city of Qom, where 1,044 centrifuges are installed, Press TV reported.
The 2015 nuclear deal, officially known as the Joint Comprehensive Plan of Action (JCPOA), allowed the first-generation IR-1 centrifuges at Fordow to spin without uranium gas.
Behrouz Kamalvandi, spokesman for the AEOI, had earlier said the injection of uranium hexafluoride is being monitored by inspectors from the International Atomic Energy Agency (IAEA).
Kamalvandi said the plant will then resume uranium enrichment, and that the level of purity of enrichment will reach 4.5 per cent by Saturday, when the inspectors will come back to check the process again.
An IAEA spokesman confirmed that UN inspectors were on the ground in Iran and would report back on relevant activities.
The fourth step in Iran's commitment reductions was initially announced by President Hassan Rouhani on Tuesday.
The President had said that Iran was ready to resume its nuclear commitments when the remaining parties to the deal fulfilled their end of the bargain.
On the first anniversary of the US withdrawal from the deal on May 8, 2019, Tehran decided to reduce its commitments to the JCPOA and set a deadline for Europeans to remedy breaches, said the state-run IRNA news agency.
( With inputs from IANS )