The International Atomic Energy Agency (IAEA) noted "with serious concern that, for over four months, Iran has denied access to the Agency... to two locations," according to an IAEA report seen by AFP.

The report said the IAEA has questions as to the possible "use or storage of nuclear material" at the two sites and that one of them "may have been used for the processing and conversion of uranium ore including fluorination in 2003".

Diplomatic sources have indicated that the IAEA's queries about the locations relate to Iran's historic nuclear programme and not its current activities.

The aforementioned site "underwent significant changes in 2004, including the demolition of most buildings", the report noted.

A third site where the IAEA has queries about the possible presence of undeclared natural uranium "underwent extensive sanitization and levelling in 2003 and 2004," according to the report.

In a separate report, the IAEA warned Iran's enriched uranium stockpile was now almost eight times the limit set in a 2015 deal.

The limit was 300 kilogrammes (661 pounds) of enriched uranium in a particular compound form, which is the equivalent of 202.8 kg of uranium.

In comparison to the latter number, the report said Iran's stockpile stood at 1,571.6 kg on May 20.

Iran has been progressively breaking the restrictions laid down in the 2015 deal in retaliation for US President Donald Trump's withdrawal from the accord in 2018 and the US's subsequent re-imposition of sanctions.

Iran reached the deal to curb its nuclear activities in return for sanctions relief with the United States -- under president Barack Obama -- Britain, France, Germany, Russia and China.