TEHRAN, Iran —  Iran's (search) intelligence chief on Wednesday accused the United States of flying spy drones over its nuclear sites and threatened to shoot down the unmanned surveillance crafts.

Intelligence Minister Ali Yunesi (search) comments backed a report in The Washington Post on Sunday that quoted unnamed U.S. officials as saying the drones have been flying over Iran for nearly a year to seek evidence of nuclear weapons programs.

"U.S. spying activities over Iranian airspace have been going since a long time ago," Yunesi told reporters.

"Most of the shining objects that our people see in Iran's airspace are American spying equipment used to spy on Iran's nuclear and military facilities," the minister told reporters.

He said they would not reveal anything the United States doesn't already know.

"Our nuclear activities are open and very transparent. Our military activities are all legal," Yunesi said. He spoke while visiting a prison for dissidents under the Shah that has been turned into a museum.

The Washington Post reported the drones had been flying over Iran from U.S. military bases in Iraq and were equipped with air filters to pick up traces of nuclear activity.

In December, the Iranian air force was ordered to shoot down any unknown flying objects. At the time, there were reports in Iranian newspapers that Iran had discovered spying devices in pilotless planes its air defense force had shot down.

"If any of the bright objects come close, they will definitely meet our fire and will be shot down. We possess the necessary equipment to confront them," Yunesi said.

Last month, Yunesi said the United States had been conducting aerial surveillance, but he neither mentioned drones nor nuclear and military sites. Yunesi said U.S. commandoes had not entered Iran, adding the security forces would catch any American who did.

Iranian media has been gripped by a kind of "flying object" fever with dozens of reported sightings in recent weeks. State-run media has reported numerous sightings of unknown objects flying over parts of Iran where nuclear facilities are located.

The objects were spotted near Natanz — which has a uranium enrichment plant — and Isfahan, which has a facility for producing uranium hexaflouride gas, the feedstock for enriching uranium.

Uranium enriched to low grades is used for fuel in nuclear reactors. Further enrichment makes it suitable for atomic bombs.

The United States accuses Iran of having a secret program to manufacture nuclear weapons. Iran denies this, saying its nuclear program is entirely for the generation of electricity.