WASHINGTON — While a national debate has erupted over the Obama administration’s lethal drone strikes overseas, federal authorities have stepped up efforts to license surveillance drones for law enforcement and other uses in U.S. airspace, spurring growing concern about violations of privacy.

The Federal Aviation Administration said Friday it had issued 1,428 permits to domestic drone operators since 2007, far more than were previously known. Some 327 permits are still listed as active.

Operators include police, universities, state transportation departments and at least seven federal agencies. The remotely controlled aircraft vary widely, from devices as small as model airplanes to large unarmed Predators.

The FAA, which has a September 2015 deadline from Congress to open the nation’s airspace to drone traffic, has estimated 10,000 drones could be aloft five years later. The FAA this week solicited proposals to create six sites across the country to test drones, a crucial step before widespread government and commercial use is approved.

Local and state law enforcement agencies are expected to be among the largest customers.

Earlier this month, TV footage showed a midsized drone circling over the bunker in southeast Alabama where a 65-year-old gunman held a 5-year-old boy hostage. After a tense standoff, an FBI team stormed the bunker, rescued the boy and shot his captor. Authorities refused to say who was operating the AeroVironment drone, which has a 9-foot wingspan.

In Colorado, the Mesa County Sheriff’s Office has used a fixed-wing drone to search for lost hikers in the mountains, and a helicopter drone to help crews battling fires. Flying manned planes or helicopters would cost at least $600 an hour, explained Ben Miller, who heads the program.

“We fly (drones) for less than $25 an hour,” Miller said. “It’s just a new way to put a camera up that’s affordable.”

Big-city police departments, including Los Angeles, have tested drones but are holding back on buying them until the FAA issues clear guidelines about operating in congested airspace, among other issues.

“You’ve got to take baby steps with this,” said Michael Downing, the LAPD deputy chief for counterterrorism and special operations.

Los Angeles Police Department officials went to Simi Valley, Calif., in December, he said, to watch a demonstration of a helicopter-like device that measured about 18 inches on each side and was powered by four propellers. It could fly about 90 minutes on its battery.

Downing said the LAPD was “pursuing the idea of purchasing” drones, but wouldn’t do so unless the FAA granted permission to fly them, and until the department could draw up policies on how to keep within privacy laws.

If the LAPD bought drones, Downing said, it initially would use them at major public events such as the Oscars or large protests. In time, drones could be flown to track fleeing suspects and assist in investigations. Tiny drones could even be used to fly inside buildings to shoot video if a suspect has barricaded himself within.

In theory, drones can offer unblinking eye-in-the-sky coverage. They can carry high-resolution video cameras, infrared sensors, license plate readers, listening devices and other high-tech gear. Companies have marketed drones disguised as sea gulls and other birds to mask their use.

That’s the problem, according to civil liberties groups. The technology is evolving faster than the law. Congress and courts haven’t determined whether drone surveillance would violate privacy laws more than manned planes or helicopters, or whether drone operators may be held liable for criminal trespassing, stalking or harassment.

“Americans have the right to know if and how the government is using drones to spy on them,” said Catherine Crump, a lawyer for the American Civil Liberties Union, which has called for updating laws to protect privacy.

A backlash has already started.

In Congress, Reps. Ted Poe, R-Texas, and Zoe Lofgren, D-Calif., introduced privacy legislation Thursday that would require police to get a warrant or a court order before operating a drone to collect information on individuals.

“We need to protect against obtrusive search and surveillance by government and civilian use,” Poe said in a telephone interview. A similar bill failed last year.

Legislatures in 15 states are considering proposals to limit drone use. The City Council in Charlottesville, Va., passed a resolution on Feb. 4 barring local police from using drones - which they don’t yet have - to collect evidence in criminal cases.

In Seattle, Mayor Mike McGinn ordered police to return two Draganflyer X6 helicopter drones earlier this month after privacy advocates and others protested. The police said they had hoped to use them for search-and-rescue operations.

Federal agencies fly drones to assist in disasters, check flood damage, do crop surveys and more. U.S. Customs and Border Protection flies the largest fleet, 10 unarmed Predators, along the northern and southern borders to help track smugglers and illegal immigrants.

In response to a question during an online Google chat Thursday, President Barack Obama said drones had never been used to kill “an American citizen on American soil.”

“The rules outside of the United States are going to be different than the rules inside the United States, in part because our capacity, for example, to capture terrorists in the United States are very different than in the foothills or mountains of Afghanistan or Pakistan,” Obama said.

No drone was sent up to help find suspected killer Christopher Dorner after his truck was found burning near Big Bear Lake in Southern California on Feb. 7, said Al Daniel, an officer in the aviation division of the San Bernardino County Sheriff’s Department. But Customs and Border Protection transmitted secure video from a Pilatus PC-12 plane to police commanders on the ground.