Robert Mueller tells Congress bureau uses drones in a 'very, very minimal way' as senators describe 'burgeoning concern'

The FBI has admitted it sometimes uses aerial surveillance drones over US soil, and suggested further political debate and legislation to govern their domestic use may be necessary.

Speaking in a hearing mainly about telephone data collection, the bureau's director, Robert Mueller, said it used drones to aid its investigations in a "very, very minimal way, very seldom".

However, the potential for growing drone use either in the US, or involving US citizens abroad, is an increasingly charged issue in Congress, and the FBI acknowleged there may need to be legal restrictions placed on their use to protect privacy.

"It is still in nascent stages but it is worthy of debate and legislation down the road," said Mueller, in response to questions from Hawaii senator Mazie Hirono.

Hirono said: "I think this is a burgeoning concern for many of us."

Dianne Feinstein, who is also chair of the Senate intelligence committee, said the issue of drones worried her far more than telephone and internet surveillance, which she believes are subject to sufficient legal oversight.

"Our footprint is very small," Mueller told the Senate judiciary committee. "We have very few and have limited use."

He said the FBI was in "the initial stages" of developing privacy guidelines to balance security threats with civil liberty concerns.

It is known that drones are used by border control officials and have been used by some local law enforcement authorities and Department of Homeland Security in criminal cases.

Mueller said he wasn't sure if there were official agreements with these other agencies.

"To the extent that it relates to the air space there would be some communication back and forth [between agencies]," Mueller said.

A Senate intelligence committee member, Mark Udall, Democrat of Colorado, later questioned whehter such use of drones was constitutional. "Unmanned aerial systems have the potential to more efficiently and effectively perform law enforcement duties, but the American people expect the FBI and other government agencies to first and foremost protect their constitutional rights," Udall said in a prepared statement.

"I am concerned the FBI is deploying drone technology while only being in the 'initial stages' of developing guidelines to protect Americans' privacy rights. I look forward to learning more about this program and will do everything in my power to hold the FBI accountable and ensure its actions respect the US constitution."

Another senator, Chuck Grassley, Republican of Iowa, also expressed concern. Asked whether the FBI drones were known about before the Mueller hearing, Grassley told CNN "absolutely not." Grassley added the FBI was asked last year whether agents were using drones but the bureau never got back with an answer.

At the same hearing, Mueller urged Congress to move carefully before making any changes that might restrict the National Security Agency programs for mass collection of people's phone records and information from the internet.

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

The FBI director argued for the continued use of the NSA programs. "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," said Mueller.

The Associated Press contributed to this report