According to an AEOI statement, the operation started at 00:00 local time (20:30 GMT) on Thursday, November 7, after the transfer of a 2,800-kilogram cylinder containing 2,000 kilograms of UF6 (uranium hexafluoride) from Natanz nuclear facility to Fordow - near the city of Qom, where 1,044 centrifuges are installed.
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.
The spokesman for the AEOI had earlier said the injection of uranium hexafluoride is being monitored by inspectors from the International Atomic Energy Agency (IAEA).
Behrouz Kamalvandi said the plant will then resume uranium enrichment, and that the level of purity of enrichment will reach 4.5 percent by Saturday, when the inspectors will come back to check the process again.
An IAEA spokesman confirmed that UN inspectors are on the ground in Iran and will report back on relevant activities.
The fourth step in Iran’s commitment reductions was initially announced by President Hassan Rouhani on Tuesday.
Rouhani confirmed in a tweet on Wednesday that the gas injection would begin later in the day.
The Islamic Republic says the suspension of its commitments is not a violation of the JCPOA but is based on Articles 26 and 36 of the agreement itself.
The fate of the Iran deal has been in doubt since May 2018, when the US abruptly abandoned the deal and reinstated the anti-Iran sanctions that it had lifted as part of the JCPOA.
Tehran says the European cosignatories — Britain, Germany and France — have so far failed to uphold their commitments. They have expressed vocal support for the deal, but failed to provide meaningful economic incentives as required under the nuclear agreement.