US President Donald Trump has accused Iran of secretly enriching uranium for a long time and said sanctions would be increased "substantially" soon.
Trump's comments came as the UN nuclear watchdog held an emergency meeting on Tehran's breach of a nuclear deal.
Washington used the session of the International Atomic Energy Agency's 35-nation board of governors to accuse Iran of extortion, after it inched past the deal's limit on enrichment levels.
The US statement maintained its offer to hold talks with Tehran.
Iran says it is reacting to harsh US economic sanctions imposed since Trump pulled out of a 2015 nuclear accord with world powers last year, and says all its steps were reversible if Washington returned to the deal.
"Iran has long been secretly 'enriching,' in total violation of the terrible 150 Billion Dollar deal made by John Kerry and the Obama Administration," Trump said on Twitter on Wednesday.
"Remember, that deal was to expire in a short number of years. Sanctions will soon be increased, substantially!"
While Iran was found to have had covert enrichment sites long before the nuclear accord, the deal also imposed the most intrusive nuclear supervision on Iran of any country.
It was not immediately clear from Trump's comments whether he was referring to previous, long-known activities or making a new allegation.
Iran's ambassador to the IAEA Kazim Gharib Abadi told reporters after Trump's accusation that all Tehran's nuclear activities were being monitored by IAEA inspectors.
"We have nothing to hide," he said after the IAEA meeting in Vienna, which was called at the request of Washington.
Abadi said in a German newspaper interview published earlier in the day that Tehran intended to preserve the nuclear deal if all other signatories honoured their commitments under it.
"Everything can be reversed within a single hour - if all of our partners in the treaty would just fulfil their obligations in the same way," he told the weekly Die Zeit.
In the past two weeks Iran has breached two limits pivotal to the 2015 deal, which aimed to extended the time Iran would need to obtain enough fissile material for a nuclear weapon, if it chose to do so, to a year from around 2-3 months.
"There is no credible reason for Iran to expand its nuclear programme, and there is no way to read this as anything other than a crude and transparent attempt to extort payments from the international community," the US mission to the IAEA said in a statement delivered at the closed-door agency board meeting.
"We call on Iran to reverse its recent nuclear steps and cease any plans for further advancements in the future.
"The United States has made clear that we are open to negotiation without preconditions, and that we are offering Iran the possibility of a full normalisation of relations."