The UN's nuclear watchdog said in a report Tuesday (March 3rd) that Iran refused access to two sites that it wished to visit in late January.
The sites were two out of three locations in relation to which the International Atomic Energy Agency (IAEA) "identified a number of questions related to possible undeclared nuclear material and nuclear-related activities", it said.
IAEA head Rafael Grossi on Tuesday sounded the alarm on Iran's nuclear programme and demanded "clarifications" over an undeclared site in Tehran where uranium particles were found late last year.
"Iran must decide to co-operate in a clearer manner with the agency to give the necessary clarifications," Grossi said.
"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."
"That worries me," Grossi added.
Further breaches reported
The IAEA has for months been pressing Tehran for information about the kind of activities being carried out at the undeclared site where the uranium particles were found.
While the IAEA has not identified the site in question, diplomatic sources told AFP the agency asked Iran about a site in the Turquzabad district of Tehran.
Grossi's warning came as the agency released a report on Iran's nuclear activities since it began breaching several parts of a landmark 2015 international deal on scaling back its nuclear programme.
The report showed its stockpile of enriched uranium now stands at more than five times the limit fixed under the deal with world powers.
As of February 19th, the report said, the Iranian stockpile stood at 1,510 kilogrammes, as opposed to the 300 kilogramme limit set under the agreement.
Some experts consider this level to provide sufficient material to produce a nuclear weapon. But it would still need several more steps, including further enrichment, to make it suitable for use in a weapon.
The report says Iran has not been enriching uranium above 4.5%.
An enrichment level of around 90% would be needed for weapons use.