A day after US Secretary of State Anthony Blinken confirmed that Iran's latest response to the nuclear deal was a step backwards, European powers expressed "serious doubts" about Iran's sincerity in seeking an agreement on its nuclear programme, warning that Tehran's position undermined prospects for a return to the 2015 deal.

While European negotiators appeared to be making progress towards a return to the 2015 agreement with Iran largely agreeing to the text of the final proposal

The degree of optimism declined when the United States sent its response, to which Iran, in turn, responded.

In a joint statement, France, Germany and the United Kingdom said that the final proposal put to Tehran represented the European Powers "maximum flexibility" on the dossier

"Unfortunately, Iran chose not to use this crucial diplomatic opportunity," she added, noting that Tehran "instead continues to escalate its nuclear programme beyond any reasonable civilian justification."

Tehran reiterated issues related to its legally binding obligations under the Treaty on the Non-Proliferation of Nuclear Weapons reached with the International Atomic Energy Agency (IAEA)

The three States reported that "the latter request raises serious doubts as to Iran's intentions and its commitment to a successful outcome with regard to the JCPOA".

"We are not about to agree to an agreement that does not meet our basic requirements," the statement from the three European powers added.