Iran says it has started moving machines used to enrich nuclear fuel to an underground site that was secret until 2009.
Iranian nuclear energy chief Fereidoun Abbasi told state television Monday the centrifuges are being transferred from Iran's Natanz nuclear site to the Fordo facility near the city of Qom.
Abbasi announced plans for the transfer in June, saying the work would be done under the supervision of the International Atomic Energy Agency.
Iran only revealed the existence of the facility – a mountain bunker – in 2009, after Western intelligence agencies detected it and said it was evidence of covert nuclear work.
Western nations have imposed a series of sanctions on Iran in an attempt to force it to halt enrichment, a process that can make weapons-grade material if done at a much higher level.
Iran denies seeking nuclear weapons and says it is enriching uranium for electricity production and medical applications.