Unmanned drones are roaming American skies conducting surveillance on people in the United States, albeit in a "very minimal way," the head of the FBI revealed to Congress on Wednesday.

Federal Bureau of Investigation director Robert Mueller said his agency's use of a small number of aerial drones is relatively new, and that the bureau has only begun to draw up policy and operational guidelines for the devices.

"I will tell you that our footprint is very small," Mueller testified before the Senate Judiciary Committee.

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

Mueller said the drones conduct surveillance, but that they were "seldom" used.

Other agencies are known to be using the high-technology gadgets, including the Department of Homeland Security, which uses drones to patrol the US border with Mexico.

Senator Chuck Grassley said Attorney General Eric Holder indicated to him in writing that the Drug Enforcement Administration and the Bureau of Alcohol, Tobacco, Firearms and Explosives "had purchased drones and were exploring their use in law enforcement."

"I think the right of privacy is at stake. If there's a legitimate law enforcement reason for using it, they ought to say," Grassley told CNN after the hearing.

He said a simmering distrust of government is fueling suspicion about domestic spying, including recently revealed surveillance programs that gather phone logs and Internet data.

"Because of that mistrust, we've got to nail these things down. The people have a right to know."

In March Republican Senator Rand Paul blocked legislative action for nearly 13 hours on the Senate floor to protest the Obama administration's refusal to unequivocally rule out drone strikes on US soil.

Days later Holder wrote to Paul clarifying that a US president does not have the power to order a drone strike against a "non combatant" American inside the United States.

Paul expressed concern about the drone surveillance, saying it should not be used without a court-issued search warrant.

"My guess is they don't have warrants for these things, they're just flying around. That, I'm opposed to," he told AFP.

Mueller did not say whether warrants were being obtained for the use of the drones.

Paul said Americans could grow fearful of drones that are small enough to land on a house window or fly indoors.

"I think there's one that weighs less than an ounce," Paul said.

Democrats have expressed concern as well. Senator Dianne Feinstein said she believed that "the greatest threat to the privacy of Americans is the drone... and the very few regulations that are on it today."

Mueller agreed that there should be public discourse over the future of the unmanned vehicles, saying "it's worthy of debate and perhaps legislation down the road."

Congress has ordered the Federal Aviation Administration to open up airspace to unmanned aircraft by October 2015.