The country's Atomic Energy Organisation said, however, that it would honour commitments to give UN inspectors access to monitor its nuclear sites.

Three European countries -- Britain, France and Germany -- have been engaged in talks to try to rescue the 2015 deal that gave Iran relief from sanctions in return for curbs on its nuclear programme.

Tensions have been escalating between Iran and the United States since May last year when President Donald Trump pulled out of the accord and began reimposing sanctions that have crippled its economy.

The arch-foes were on the cusp of confrontation in June when Iran downed a US drone and Trump ordered retaliatory strikes before cancelling them at the last minute.

On Saturday, Iran's Atomic Energy Organisation said it had activated 20 IR-4 and 20 IR-6 centrifuges as its latest step back in rolling back its commitments.

"The centrifuge machines, as they are engaged in research and development, will help with increasing the stockpile," said the agency's spokesman, Behrouz Kamalvandi.

"The capacity of these machines is many times more than the previous machines. This started as of yesterday (Friday)," he told a news conference in Tehran.

But Kamalvandi said Iran would allow the International Atomic Energy Agency (IAEA) to continue monitoring its nuclear programme, as it has done under the 2015 accord.

"Regarding the monitoring and accesses of the IAEA... so that everything is clear (Iran's) commitments regarding transparency will be followed as before," he said.

- Europe 'must hurry' -

The European Union on Friday emphasised its reliance on the UN nuclear watchdog to monitor Iran's activities as it voiced "great concern" over the country's decision to roll back its commitments.

The IAEA in its latest report, on August 30, said it continues to verify compliance through cameras and on-site inspections.

But in an apparent hint at worries about access it said "ongoing interactions ... require full and timely cooperation by Iran".

The latest move by Iran came after EU members Britain, France and Germany were unable to find a way to offset the impact of sanctions on the country before a September 7 deadline set by the Islamic republic.

"If Europe wants to do something, it must hurry, because returning to the situation before reducing commitments could take time," Kamalvandi said on Saturday.

Tehran has already hit back twice with countermeasures in response to the US withdrawal from the 2015 deal.

On July 1, Iran said it had increased its stockpile of enriched uranium to beyond the 300-kilogram maximum set by the deal.

A week later, it announced it had exceeded a 3.67-percent cap on the purity of its uranium stocks.

On Saturday, however, Iran indicated it had no plans to step up the enrichment of uranium to higher levels.

"We currently do not need 20 percent enrichment, and if we do so at some time, we will first increase the 4.5 percent stockpile and then act," said Kalamvandi.

The announcement came on the eve of a visit to Iran by the acting head of the IAEA, Cornel Feruta.

Kamalvandi said Feruta would meet the head of Iran's Atomic Energy Organisation, Ali Akbar Salehi, and Foreign Minister Mohammad Javad Zarif.