One of the pigeons was caught near a rose water production plant in the city of Kashan in Isfahan province, the Etemad Melli newspaper reported. It said that some metal rings and “invisible” strings were attached to the bird, suggesting that it might have been somehow communicating what it had seen with the equipment it was carrying.

“Early this month, a black pigeon was caught bearing a blue-coated metal ring, with invisible strings,” a source told the newspaper.

The source gave no further description of the pigeons, nor what their fate might be.

Natanz is home to Iran’s heavily-bunkered underground uranium enrichment plant, which is also not far from Kashan.

The activity at Iran’s controversial uranium enrichment facility is the focus of Iran’s five-year standoff with the West, which fears it aims to develop nuclear weapons. The Tehran government insists its programme is intended to generate power for civilian use only.

Last year, Iran issued a formal protest over the use of espionage by the United States to produce a key intelligence report on the country’s controversial nuclear programme.

It is also highly suspicious of Israel, whose extensive intelligence activities are not known to include the use of pigeons.