President Hassan Rouhani officially inaugurated the cascades of 164 IR-6 centrifuges and 30 IR-5 devices at Iran's Natanz uranium enrichment plant in a ceremony broadcast by state television.

The television aired no images of the cascades but broadcast a link with engineers at the plant who said they had introduced uranium hexafluoride gas to the cascades after receiving the order from Rouhani.

Iran's latest move to step up uranium enrichment follows an opening round of talks Tuesday with representatives of the remaining parties to the nuclear deal on bringing the United States back into the deal.

Former President Donald Trump withdrew from the deal in 2018.

The Vienna talks are focused not only on lifting crippling economic sanctions Trump reimposed, but also on bringing Iran back into compliance after it responded by suspending several of its own commitments.

All sides said the talks, in which Washington is not participating directly but has the European Union as intermediary, had got off to a good start.

The IR-5 and IR-6 centrifuges allow uranium to be enriched more quickly and in greater amounts than the Iran's first generation devices, which are the only ones that the 2015 deal allows it to use.

Rouhani again underlined at the ceremony, which coincided with Iran's National Nuclear Technology Day, that Tehran's nuclear programme is solely for "peaceful" purposes.