WASHINGTON – FBI Director Robert Mueller acknowledged Wednesday that the bureau has used unmanned aerial drones for surveillance in the United States and suggested that government needs to develop guidelines as their use grows.

Testifying before the Senate Judiciary Committee, Mueller said the bureau has employed drones in “a very, very minimal way and very seldom.”

“I will tell you that our footprint is very small,” he said, responding to a question by Sen. Charles Grassley, R-Iowa. “We have very few and have limited use, and we’re exploring not only the use but also the necessary guidelines for that use.”

Mueller’s comments come amid a growing public debate over privacy and civil liberties spurred by the recent revelation of the National Security Agency’s massive telephone and Internet surveillance program.

Chris Calabrese, legal counsel for the American Civil Liberties Union, called Mueller’s acknowledgement disturbing. Jim Harper, director for information policy studies at the libertarian-leaning Cato Institute, said that domestic surveillance drone use raises Fourth Amendment search and seizure questions.

“I think it’s something that the FBI has started to use these drones without any clear policy to protect our privacy,” Calabrese said. “Now we have cheap, easily accessible technology to allow law enforcement to spy on us. Law enforcement should only use these drones if they have probable cause.”

Sen. Dianne Feinstein, D-Calif., who chairs the Senate intelligence committee and supports the NSA surveillance programs, echoed Calabrese’s concerns.

“If people are concerned about privacy, I think the greatest threat to privacy of Americans is the drone and the use of the drone, and the very few guidelines that are on it today and the booming industry of commercial drones,” she said.

In March, Sen. Rand Paul, R-Ky., waged a 13-hour filibuster railing against CIA Director John Brennan’s nomination in which he warned of increasing government use of aerial drones on U.S. soil.

After his filibuster ended, Paul received a letter from Attorney General Eric Holder stating that President Obama doesn’t have the power to “use a weaponized drone to kill an American not engaged in combat on American soil.”