The US uses drones for surveillance in some limited law-enforcement situations, the head of FBI has said, prompting additional debate about the Obama administration's use of domestic surveillance.

Robert Mueller's acknowledgement came in response to questions on Wednesday from members of the Senate Judiciary Committee 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.

"I will tell you that our footprint is very small," Mueller said in his testimony.

"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."

Privacy implications

At the 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," she said.

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

The FBI used a drone during a hostage-taking in Alabama this year after an armed, 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 US border with Mexico.

The Obama administration has been defending its surveillance tactics since Edward Snowden, a former National Security Agency (NSA) contractor, released secret documents revealing a huge database of daily telephone records, as well as coordination between the NSA and social media companies.

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

Drone use defended

In a speech last month, President Barack Obama defended the use of armed drones abroad but said the US 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.

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

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

In March Republican Senator Rand Paul blocked legislative action for nearly 13 hours on the Senate floor to protest against 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 US.

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

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

Surveillance defended

Mueller also urged Congress to move carefully before making any changes that might restrict the NSA programmes for mass collection of people's phone records and information from the internet.

He said there are 10 or 12 cases in which the phone-records programme contributed to breaking up violent plots.

Mueller said communications capabilities of plotters are their weakest link.

"If we are to prevent terrorist attacks, we have to know and be in their communications," he said.

"Having the ability to identify a person in the United States, one telephone number with a telephone that the intelligence community is on in Yemen or Somalia or Pakistan ... may prevent that one attack, that Boston or that 9/11."

He was referring to a bombing at the Boston marathon and the attacks using hijacked airliners against the US on September 11, 2001.

Mueller argued for the continued use of the NSA programmes.

"Are you going to take the dots off the table, make it unavailable to you when you're trying to prevent the next terrorist attack? That's a question for Congress," he said.