WASHINGTON  - The United States uses drones for surveillance in some limited law enforcement situations, FBI Director Robert Mueller said on Wednesday, sparking additional debate about President Barack Obama's use of domestic surveillance.

The acknowledgement came in response to questions from US senators who said they wanted to know more about the federal government's increasing use of unmanned aircraft.

"Does the FBI use drones for surveillance on US soil?" Republican Senator Charles Grassley of Iowa asked during a Senate Judiciary Committee hearing.

"Yes," Mueller said, adding that the use was in "a very, very minimal way and very seldom."

Mueller did not go into detail, but the FBI later released a statement that said unmanned aircraft were used only to watch stationary subjects and to avoid serious risks to law enforcement agents. The Federal Aviation Administration approves each use, the statement said.

The FBI used a drone during a hostage-taking in Alabama this year after a gunman, Jimmy Lee Dykes, snatched a boy off a school bus and held him in an underground bunker, according to the statement.

The US government has made no secret of its use of drones to monitor the United States border with Mexico.

The Obama administration has been defending its surveillance tactics since former National Security Agency 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.

The programs are designed to target militants outside the United States who are suspected of planning attacks, but they inevitably gather some data on Americans, US officials said.

In a May speech, Obama defended the use of armed drones abroad but said the United States should never deploy armed drones over US soil.

The Justice Department had disclosed that two domestic law enforcement agencies use unmanned aircraft systems, according to a department statement sent to the Judiciary Committee and released on Wednesday by Grassley's office. The two are the Drug Enforcement Administration and the Bureau of Alcohol, Tobacco, Firearms and Explosives.

Grassley sent a letter to Attorney General Eric Holder on Wednesday asking why the Justice Department did not earlier mention the FBI's use of drones.

At Wednesday's hearing, Democratic Senator Dianne Feinstein of California said she was concerned about the privacy implications of drone surveillance.

"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," Feinstein said.

Mueller reiterated that drone use is rare. "It is very narrowly focused on particularized cases and particularized needs," he said.

Mueller is due to retire when his term expires in September.