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 unmanned surveillance vehicles (UAVs) the FBI has or how often they have been used.
But a law enforcement official told CNN the FBI has used them a little more than a dozen times but did not say when that started. The official said drones are useful in hostage and barricade situations because they operate more quietly and are less visible than traditional aircraft such as helicopters.
The FBI said it used a UAV earlier this year to monitor the situation where a boy was held hostage in a bunker in Alabama.
Bureau spokesman Paul Bresson said their use allows "us to learn critical information that otherwise would be difficult to obtain without introducing serious risk to law enforcement personnel."
Bresson said the aircraft can only be used to perform surveillance on stationary subjects and the FBI must first get approval from the Federal Aviation Administration to fly in a "very confined geographic area."
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."
Sen. Mark Udall, a Colorado Democrat, said he was concerned the FBI was deploying drone technology and only in the initial stages of developing guidelines "to protect Americans' privacy rights."
Grassley wants answers from Holder
Grassley sent a letter to Attorney General Eric Holder following the hearing asking why written information Holder sent him last month about federal law enforcement use of drones included related information about the Bureau of Alcohol Tobacco Firearms and Explosives and the Drug Enforcement Administration, but not the FBI.
Grassley sought answers to several questions and asked Holder to reply by June 28. He wants to know who at the FBI authorized drone use and information on the uses and limitations of their use and whether any are armed or capable of being armed.
The Justice Department said it was reviewing Grassley's letter.
Mueller said Wednesday the drones were used for surveillance.
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.