Iranian civilians began reporting seeing coloured flashes and racing lights in the sky in December, sparking speculation of UFOs.

But Iranian air force commanders, many of them trained in the US, identified the drones, and Iran's National Security Council decided not to engage them, the report said.

US officials familiar with the program told the daily the drones use radar, video, still photography and air filters to scan for evidence of nuclear activity not accessible to satellites.

"The aerial espionage is standard in military preparations for an eventual air attack and is also employed as a tool for intimidation," the Post said.

The drones were deployed along Iran's northern and western borders in April, December and January, the report said. In response Iran's military stepped up defenses around its nuclear facilities in southern Isfahan and Bushehr.

But Iran did not take the bait and turn on its radar, which would have revealed vital information about the country's air defence systems, the Post said.