Vienna - The United States has accused Iran of trying to "extort" money from the international community by violating the 2015 nuclear accord.
The comments were made by Washington's ambassador to the International Atomic Energy Agency (IAEA), a global watchdog that has been monitoring Iran's nuclear activity under the increasingly fragile deal.
"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," Jackie Wolcott told the agency's 35-nation board of governors in Vienna.
She was referring to two recent breaches of the deal by Iran: Tehran is now enriching uranium beyond the agreed level, while also increasing its stockpile of low-enriched uranium above the agreed limit.
Such limits were designed to prevent the Islamic Republic from developing a nuclear bomb, in return for the lifting of sanctions.
President Donald Trump announced the United States' withdrawal from the Obama-era agreement last year and reimposed sanctions against Tehran as part of a "maximum pressure" approach.
"Sanctions will soon be increased, substantially!" Trump threatened via Twitter later Wednesday, while also claiming without evidence that Iran "has long been secretly 'enriching.'"
In Vienna, Wolcott repeated that Washington is "open to negotiation without preconditions" and called on Tehran to reverse the recent violations.
"We are offering Iran the possibility of a full normalization of relations," she said.
However, Iran has refused any such offer to renegotiate the Joint Comprehensive Plan of Action (JCPoA), as the accord is formally known.
Despite efforts by European countries to salvage the deal - including an EU transaction system known as Instex that allows Iran to circumvent US sanctions and continue doing business with international partners - Iran announced that it too would partially withdraw from the accord in May.
The violations by Tehran are intended to compel European partners to step up efforts to help Iran's sanctions-hit economy or face the death of the deal, which took years to negotiate.
But Washington is also trying to persuade European allies to be tougher on Iran. The Trump administration argues the deal did not do enough to constrain Iran's nuclear and missile programmes or check Tehran's violent activities in the Middle East.
Germany, France, Britain and the European Union expressed "deep concern" on Tuesday at Iran's enrichment activities and called on Tehran to reverse the move "without delay."
At the UN nuclear watchdog's special meeting on Wednesday, called by the US, representatives were to be briefed by IAEA leader Yukiya Amano on the latest findings regarding Iran's nuclear programme. Agency inspectors have access to Iranian facilities and issue quarterly reports on the programme's status.
No major decisions are expected to be announced following the meeting.
Tehran said it was glad the IAEA was meeting, describing it as "a good opportunity for Iran to explain its position and challenge the parties that have failed to implement the nuclear deal."
"It's black humour on the part of the US to demand a meeting about an agreement from which they themselves left," Foreign Ministry spokesman Abbas Mousavi told state news agency IRNA on Wednesday.
Mousavi said Iran wanted to stay in the deal but needed to see the promised economic benefits. "We are still open for diplomacy - but no lip service about the atomic deal - instead concrete and practical solutions."
He said Iran was not open to renegotiating the accord.dpa