The UN's atomic watchdog has concluded that Iran has nearly tripled its stockpile of low-enriched uranium over the past three months. If that is the case, then Tehran can be accused of a sharp violation of the 2015 nuclear deal signed with six world powers.

Some experts have said Iran is reaching a volume sufficient for the production of a nuclear weapon, but they point out that it would require several more steps—including further enrichment—to make the uranium suitable for use in a bomb. According to the Washington-based Arms Control Association, Iran would need roughly 1,050kg of low-enriched uranium—under 5% purity—and would then need to enrich it further to weapons-grade, or more than 90% purity, for a weapon.

In a March 3 report, the UN’s International Atomic Energy Agency (IAEA) said that Iran’s stockpile of low-enriched uranium had expanded to 1,020.9 kilograms compared to the nuclear accord limit of 300 kilograms. In its previous report, released in November, the watchdog said the stockpile stood at 372.3 kilograms.

The IAEA also said that Iran appears to have three undeclared atomic sites. It said it was demanding “clarifications” from Tehran. It added that Tehran refused to grant access to two of three suspected nuclear sites that the UN agency wanted to visit in late January.

Iran has insisted that its nuclear programme remains strictly for civilian and energy uses and that it has never had any ambition to build a nuclear bomb.

IAEA chief Rafael Grossi told AFP that "Iran must decide to cooperate in a clearer manner with the agency to give the necessary clarifications.

"The fact that we found traces [of uranium] is very important. That means there is the possibility of nuclear activities and material that are not under international supervision and about which we know not the origin or the intent," Grossi said.

"That worries me," he added.

The US unilaterally abandoned the multilateral nuclear deal in 2018 even though Iran in the UN’s eyes was in full compliance with it.

Iran has said it no longer considers itself bound by the agreement and has gradually reduced some of its commitments under the deal.

It has said it would be willing to move back to full compliance with the deal if Europe provides “meaningful” economic benefits in the face of US sanctions that are designed for force Tehran into a much tougher nuclear deal.

With the nuclear deal in place, Iran’s so-called “breakout time”—the time Tehran would require to build a bomb if it chose to do so—stood at around a year. Since Iran started ignoring limits of the nuclear deal, it has slowly shortened that period.

Prior to the nuclear deal, Iran enriched uranium up to 20%, meaning it was a short technical step away from weapons-grade levels of 90%. In 2013, Iran’s stockpile of enriched uranium was assessed at more than 7,000kg with higher enrichment.