The long-standing director of the FBI has admitted the agency is using drones for surveillance in American skies.

FBI director Robert Mueller, who took over as the director of the FBI just a week before the September 11 attacks, made the acknowledgment in response to questions from US senators asking about the government's increasing use of unmanned aircraft.

During a Senate Judiciary Committee hearing, Republican Senator Charles Grassley of Iowa asked Mr Mueller if the agency was using drones "for surveillance on US soil".

Mr Mueller said drones were being used, but said their use was in "a very, very minimal way and very seldom".

"Our footprint is very small," he said.

Key points: FBI director Robert Mueller admits the agency is using drones for surveillance on US soil

FBI director Robert Mueller admits the agency is using drones for surveillance on US soil Mr Mueller says drone use is "very seldom" and they are being used in "a very, very minimal way"

Mr Mueller says drone use is "very seldom" and they are being used in "a very, very minimal way" Announcement as Obama administration defends its surveillance tactics following Edward Snowden's leaks

Announcement as Obama administration defends its surveillance tactics following Edward Snowden's leaks Democrat Senator Dianne Feinstein says drones are the "greatest threat to the privacy of Americans"

"We have very few and of limited use and we're exploring not only the use but also the necessary guidelines for that use."

The FBI later released a statement saying unmanned aircraft were used only to watch stationary subjects and to avoid serious risks to law enforcement agents, saying each use was approved by the US Federal Aviation Administration.

Speaking on Wednesday, Mr Mueller said he could not exactly say what images, if any, were being gathered from the drones and conceded that there is a gap in the law when it comes to protecting the privacy of US citizens from the use of drones by the private sector.

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"We already have, to a certain extent, a body of law that relates to aerial surveillance and privacy relating to helicopters and small aircraft ... [which could] be adapted to the use of drones but it's still in its nascent stages," he said.

"But it is worthy of debate and perhaps legislation down the road."

'Greatest threat to privacy of Americans'

Drones have been used to patrol US borders, search for missing children, to provide a bird's eye view in hostage situations or bushfires, and by some police departments, but this is the first time the FBI has publicly stated that they are being used to spy on Americans.

The Obama administration has been defending its surveillance tactics since former National Security Agency (NSA) contractor Edward Snowden released secret documents revealing a massive database of daily telephone records, as well as coordination between the NSA and social media companies.

Democrat Senator Dianne Feinstein, who has been one of the loudest voices in Washington over the past week supporting the secret phone and internet surveillance program leaked by Snowdon, says drones are the "greatest threat to the privacy of Americans".

"If people are concerned about privacy, I think the greatest threat to the privacy of Americans is the drone and the use of the drone and the very few regulations that are on it today," she said.

The American Civil Liberties Union says drones, like any other surveillance tool, should only be used with a warrant and that any images they record should only be kept if there is reasonable suspicion they contain evidence of a crime, or are relevant to an ongoing criminal investigation.

ABC/Reuters