President Vladimir Putin on Monday eased restrictions on Russian companies working on Iranian enrichment sites as he travelled to Tehran for his first visit since 2007.
A decree Putin signed on Monday enables Russian firms to help modify centrifuges at the Fordo enrichment site and help Tehran redesign its Arak heavy water reactor.
Russian companies can now also carry out activities linked to Iranian exports of enriched uranium of more than 300 kilograms in exchange for the supplies of natural uranium to Iran, the Kremlin decree said.
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Under a historic July deal with world powers, Iran agreed to dramatically scale back its nuclear programme, making it much more difficult for it to develop nuclear weapons.
Tehran agreed to slash by two-thirds the number of centrifuges, machines that can "enrich" or purify uranium to make it suitable for peaceful uses but also for a nuclear weapon.
Russian companies are eyeing business opportunities after sanctions on Iran are lifted, expected in the next two months, as the nuclear deal reaches its "implementation" stage.
Putin arrived in Tehran on Monday for talks with supreme leader Ayatollah Ali Khamenei and President Hassan Rouhani, with the Syrian conflict expected to be high on the agenda.
The one-day visit will also see Putin take part in a summit of gas exporting countries.