WASHINGTON (CNN) -- FBI Director Robert Mueller acknowledged the law enforcement agency uses drone aircraft in the United States for surveillance in certain difficult cases.
Mueller told the Senate Judiciary Committee on Wednesday that drones are used by the FBI in a "very, very minimal way and very seldom." He did not say how many drones the FBI has or how often they have been used.
A federal law enforcement official said the aircraft have been used for surveillance in hostage situations and also when suspects have taken refuge behind barricades.
"We use it sparingly in dangerous situations where the risk to agents lives are at stake," the official told CNN.
The FBI previously acknowledged using unmanned surveillance aircraft this year to monitor the situation where a boy was held hostage in a bunker in Alabama.
Mueller's comments come as the Obama administration grapples with political and other fallout from the public disclosure of top-secret surveillance programs, which has triggered new debate over reach of national security vs. privacy rights.
National security and law enforcement officials have defended National Security Agency telephone and e-mail surveillance of overseas communications as an effective tool in fighting terror.
President Barack Obama has assured Americans the government is not listening to their phone conversations or reading their e-mail.
But Sen. Charles Grassley, an Iowa Republican, asked Mueller whether the FBI had guidelines for using drones that would consider the "privacy impact on American citizens."
Mueller replied the agency was in the initial stages of developing them.
"I will tell you that our footprint is very small," he said.
Senate Intelligence Committee Chairman Dianne Feinstein expressed concern over drone use domestically.
"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 and the booming industry of commercial drones," the California Democrat said.
Mueller said he would need to check on the bureau's policy for retaining images from drones and report back to the panel.
"It is very narrowly focused on particularized cases and particularized needs and particularized cases," said Mueller. "And that is the principal privacy limitations we have."
Members of Congress and privacy advocates have pressed for regulations on the use of drones, and their use in counterterror operations overseas was a controversy that flared publicly during confirmation hearings for CIA Director John Brennan earlier this year.
Unmanned drone use is becoming more common in the United States although it is not lawful in many cases. The Federal Aviation Administration forecasts some 10,000 civilian drones will be in use in the United States within five years, including those for law enforcement and commercial purposes.
Amie Stepanovich, of the Electronic Privacy Information Center, has previously said law enforcement should not use drones as an alternative to police patrols.
She said that they should be used for specific operations and that Congress should pass a law requiring legal permission. And a proposal in the U.S. House would require law enforcement to obtain a search warrant before using drones to investigate crimes.
Because they are cheaper to use than helicopters, unmanned aircraft can be used to monitor crops and livestock, look at damage to buildings and for other uses.
The FAA recently announced plans to create six drone test sites around the country.