Israeli Prime Minister Benjamin Netanyahu shows a photo of Iran’s “secret atomic warehouse”. (Photo: Reuters)

The International Atomic Energy Agency (IAEA) is investigating the particles’ origin and has asked Iran to explain the traces. But Tehran has not yet done so, according to the diplomats, stoking tensions between Washington and Tehran.

Reuters first reported in April that the IAEA, which is policing the nuclear deal, had inspected the site - a step which was to be taken “only when necessary” - and  samples gathered there were sent off for analysis.