European mediators last month appeared to make progress in restoring the 2015 accord as Iran largely agreed to a proposed final text.

But optimism dimmed when the United States sent a reply, to which Iran in turn responded.

In a joint statement on Saturday, France, Germany and Britain said that the final package put to Tehran had taken the European powers "to the limit of their flexibility"

"Unfortunately, Iran has chosen not to seize this critical diplomatic opportunity, they said, assessing that "instead, Iran continues to escalate its nuclear programme way beyond any plausible civilian justification".

Tehran has also reopened issues related to its legally binding obligations under the Non Proliferation Treaty that was concluded with the UN atomic energy watchdog IAEA.

"This latest demand raises serious doubts as to Iran's intentions and commitment to a successful outcome on the JCPoA," France, Germany and Britain said, using the acronym for the 2015 Joint Comprehensive Plan of Action.

The JCPoA had been trashed by former US president Donald Trump, who instead imposed sweeping new sanctions.

Hopes had grown that the agreement might be revived under President Joe Biden who favours restoring it.

Under the deal, Iran will enjoy sanctions relief and again be able to sell its oil worldwide in return for tough restrictions on its nuclear programme.

The three European powers' statement came a day after US Secretary of State Antony Blinken assessed that Iran's latest reply on the nuclear deal is a step "backwards".

He also warned that Washington is "not about to agree to a deal that doesn't meet our bottom-line requirements".