The report from the International Atomic Energy Agency (IAEA), seen by AFP, says: "The agency detected natural uranium particles of anthropogenic origin at a location in Iran not declared to the agency."

While the IAEA itself has not named the site in question, diplomatic sources have previously said the agency has been posing questions to Iran relating to a site where Israel has alleged secret atomic activity in the past.

Sources say the IAEA took samples from the site in the Turquzabad district of Tehran in the spring.

The report also confirms that Iran has ramped up uranium enrichment, with the stockpile of enriched uranium now reaching the equivalent of 551 kilogrammes, as opposed to the 300-kilogramme limit laid down in Iran's 2015 deal with world powers.