Mohammad Eslami, head of the Atomic Energy Organisation of Iran, was speaking as talks in Vienna on bringing Washington back into the 2015 agreement abandoned by president Donald Trump reach a critical stage.

The 2015 deal set a 3.67 percent limit for Iran's uranium enrichment, sufficient for its power generation needs although not for some other civilian uses, such as the production of medical isotopes.

Since Trump abandoned the agreement and reimposed crippling sanctions, Iran has responded by producing uranium enriched to 60 percent, a level that has sparked Western concern although it still falls short of weapons grade.

Iran credits its response with bringing President Joe Biden's administration back to the negotiating table for on-off talks that have now lasted 10 months.

"Enrichment can be done at any moment when (officials) decide or there is the will for it," Eslami said on his organisation's website.

"If they [the US] do not fulfil their obligations, we will return to the previous situation.

"Enrichment is in place in the country with a maximum ceiling of 60 percent and this brought the West to negotiations."

During the negotiations in Vienna, Iran has repeatedly called for guarantees from the Biden administration that there will be no repeat of Trump's 2018 walkout, something that partisan divisions on Iran policy have effectively precluded.