Image caption Robert Mueller said the FBI used drones for surveillance in "particular incidents"

The FBI has used drones for surveillance in limited cases over US soil, FBI Director Robert Mueller has told a US Senate committee.

Mr Mueller said the agency had "very few" drones and had used them in "a very minimal way" and "very seldom".

But the director said the FBI was in the "initial stages" of developing drone policies.

The revelation comes amid a debate about how the US government balances privacy and national security.

It recently emerged that America's electronic spying agency has been harvesting US phone records and overseas internet data.

Mr Mueller described the drone use in testimony on Wednesday to the US Senate judiciary committee. He was questioned on the matter by Iowa Republican Chuck Grassley.

"I will tell you that our footprint is very small," said Mr Mueller, who is retiring in September after 12 years as FBI director.

He said drones were used in "particular incidents where you need the capability", adding he was unsure how long images captured by the drones were kept.

Image caption US civil rights activists protest outside a recent international conference on drones

The panel's chairwoman, Sen Dianne Feinstein, told the FBI director she believes that unmanned planes are the biggest threat to Americans' privacy, especially their use by private firms.

The authorities used a surveillance drone during a February stand-off with an Alabama man who shot dead a school bus driver and then took a five-year-old boy hostage, according to media reports at the time.

Congress has ordered the Federal Aviation Administration to open up US airspace to unmanned aircraft by October 2015.

The aerospace industry predicts there will be 30,000 drones deployed globally within five years, half of them in the US, the Associated Press news agency reports.

In March, a Republican senator raised concerns about the use of drones against Americans.

Kentucky's Rand Paul demanded a pledge that the federal government would not use such aircraft in the US to kill terror suspects who are American citizens.

Sen Paul spoke without a pause on the issue for nearly 13 hours on the Senate floor, in a tactic known as a filibuster, to delay the nomination of a new CIA chief.

In May, US President Barack Obama said he would curtail the use of armed drones in operations outside the US.

Under the new policy described by the White House, the US will only allow drones to be used in areas that are not overt war zones when there was a "continuing, imminent threat" to the US and capture was not feasible.