The US, Britain, Germany and France have told the UN's nuclear watchdog that Iran needs to take “essential and urgent actions” to clear up outstanding issues concerning its nuclear material.
The countries called on Tehran on Thursday to fully comply with a November 2022 resolution and what was agreed on in a joint statement on March 4.
They demanded clarification over Iran's nuclear material and said the International Atomic Energy Agency's board would “have to be prepared to take further action in support of the Secretariat to hold Iran accountable in the future, including the possibility of a resolution”.
The intervention by the four countries known as "the Quad" comes after IAEA chief Rafael Grossi made clear his disappointment at the lack of progress since he visited Tehran in March.
Mr Grossi says the IAEA cannot verify whether Iran's nuclear programme has peaceful intentions until it explains the presence of uranium traces at two sites.
Tehran was urged by the Quad to act without delay to resolve all outstanding issues.
“Iran needs to provide, without further delay, technically credible information on the current location(s) of nuclear material and contaminated equipment in relation to Turquzabad and Varamin,” the statement by the four countries read.
Iran has not addressed the presence of uranium particles of anthropogenic origin found by the agency in Turquzabad and Varamin, the statement added.
“We note that the agency’s assessment of the activities undertaken in Marivan remains as set out previously: That Iran conducted explosive experiments in preparation for the use of neutron detectors at this site,” the four countries said.
“This is a stark reminder of why we need to continue to demand full transparency from Iran on all outstanding safeguards issues,” they said.
Mr Grossi's visit in March ended with a joint statement in which Iran said it was ready to "provide further information and access" in a "spirit of collaboration". However, he told the IAEA board this week that there had been "no further progress" since initial steps to restore UN surveillance.
The tussle has cast a shadow over stalled diplomatic efforts to bring Iran back into compliance with a 2015 nuclear deal. Since the US quit the pact, Tehran has enriched uranium far beyond what it agreed to, and to a level that western powers say has no credible civilian purpose.
The nuclear watchdog still awaits information on a discrepancy detected more than a year ago between the amount of natural uranium at Jaber Ibn Hayan Laboratory declared by Tehran and the amount verified by the agency, the statement said.
The agency said Iran’s account was “not to be based on scientific grounds and, therefore, not acceptable”.
“We call upon Iran to engage the agency to explain the shortfall of nuclear material and demonstrate ‘valid and technically sound measurement results’,” the statement said.
“It is worth recalling that this material is related to a previous safeguards site of concern – LavisanShian.”