"Negotiations are underway between Tehran and Moscow to construct the second and third units of the Bushehr power plant," said Behrouz Kamalvandi, spokesman for the Atomic Energy Organization of Iran (AEOI).

The move was "in accordance with the Islamic republic's plans to produce at least 10,000 megawatts of electricity using nuclear energy", he said, quoted on the government's website.

The announcement coincides with ongoing talks in Vienna seeking to revive a 2015 nuclear deal between Iran and world powers, including Russia.

The parties to the accord that gave Iran relief from sanctions in return for curbs on its nuclear programme saw it as the best way to stop Tehran from building a nuclear bomb -- a goal it has always denied.

The United States unilaterally withdrew from the deal and began imposing sanctions on Iran in 2018 under then-president Donald Trump, prompting Tehran to begin walking back on its commitments under the agreement.

Moscow built the Bushehr plant, equipped with a 1,000 MW reactor, delivering it in September 2013 after years of delay.

Kamalvandi said the project was being implemented "without delay", despite payments being behind schedule.

The announcement immediately follows a two-day visit by ultraconservative Iranian President Ebrahim Raisi to Moscow, during which he said Tehran has "no limits for expanding ties with Russia".

Iran has substantial reserves of oil and natural gas, but it hopes to build 20 nuclear power plants on the long term in a bid to reduce its dependence on fossil fuels.

In late December, the deputy head of the AEOI, Mahmoud Jafari, said there was a two-year delay in the construction of new units due to "financial problems".