The FBI is operating "a small air force" of planes equipped with video and cellphone surveillance technology, hidden behind fictitious companies that are fronts for the government, the Associated Press revealed on 2 June.

In a report, the wire service described how it has traced at least 50 surveillance aircraft linked to the FBI, and more than 100 flights over at least 11 states since late April.

The planes flew over both rural and urban areas, including Houston, Phoenix, Seattle, Chicago, Boston, Minneapolis and Southern California.

While low-flying planes can capture video of unrelated criminal activity on the ground that could be handed over for prosecutions, some of the aircraft can be equipped with technology that can identify thousands of people below through the cellphones they carry, even if they are not making a call or in public, the AP said.

The intelligence and security bureau told the AP it uses the planes only in ongoing investigations.

"[The planes] are not equipped, designed or used for bulk collection activities or mass surveillance," Christopher Allen, the FBI's spokesman, told the AP.

The planes' surveillance equipment is generally used without a judge's approval.

Aviation program 'not a secret'

While the government has been using fake companies, they said the surveillance program was not being hidden.

"The FBI's aviation program is not secret," Allen said.

However, it has been reported that the FBI did ask the AP not to disclose the names of the front companies, saying it would force the bureau to create new ones, which would come at a cost to taxpayers.

The AP declined the request, and published some of the names of the 13 fake companies, including FVX Research, KQM Aviation, NBR Aviation and PXW Services.

Even basic aspects of the program are withheld from the public in censored versions of official reports from the Justice Department's inspector general, the AP added.

The FBI does not typically seek a judge's approval before employing the video snooping gear, but recently began obtaining court orders for stingray use due to new policies.

In March, an investigation by The Wall Street Journal uncovered another programme, linked to the CIA, that is capable of collecting data on thousands of cellphones per flight.

President Barack Obama, meanwhile, has said he welcomes a debate on government surveillance, and has called for more transparency about spying in the wake of disclosures about classified programs.