The Federal Aviation Administration (FAA) said Thursday that the process of authorizing drone aircraft use in the United States faces significant hurdles and is behind schedule. The declaration — part of a long-term FAA "roadmap" on the issue — comes amid growing privacy concerns associated with the potential widespread use of unmanned aircraft by civilians and officials.

In 2012, Congress directed the FAA to grant drones widespread access by September 2015, a deadline the agency said it cannot meet. For the time being, domestic drone use will take place only in six unidentified test sites. For the next several years, the agency said, the use of drones will require permits granted by the FAA, on a case-by-case basis, to operators who agree to safety procedures.

The FAA’s 74-page “roadmap” for drone authorization briefly mentions the privacy issue, saying the agency will take steps to make sure any policy governing the use of pilotless aircraft “addresses privacy concerns.”

"The FAA's mission does not extend to regulating privacy, but we have taken steps to address privacy as it relates to the six ... test sites," the agency said in response to questions from The Associated Press.

Privacy advocates and civil rights groups say there is great public concern about the widespread use of drones, or unmanned aerial vehicles.

“We think what is important is that it’s good that they are requiring procedural privacy protections, and that they are sharing that information publicly,” said Chris Calabrese, legislative council for the American Civil Liberties Union (ACLU). “But at some point they are going to have to move from process to substance.”

Calabrese wants to see the FAA establish substantive policies and limitations on the use, collection and retention of information that could potentially be gathered by drones — things the ACLU mentioned in a 2011 recommendation on use of the craft.

“The FAA needs to limit how information is collected and shared,” he said. “If you’re flying a drone to spread pesticide on your crops, you don’t need to be collecting any property records or personal information. Similarly, if you’re a drone looking for downed power lines, you shouldn’t be taking pictures of people in their backyard, and if you do come across that information you need to destroy it immediately.”

Jeramie Scott, a national security fellow at the Electronic Privacy Information Center, agreed.

“One of the issues is you’re having a piece of equipment that can fly with lots of advanced surveillance equipment, including HD cameras, potential facial recognition, license plate recognition and infrared cameras,” Scott said. “There are a lot of good uses for drones, there is no doubt about that. But the privacy issues should be addressed up front.”

At the six test sites, the FAA can evaluate the ability of remote-controlled drones to detect and avoid other aircraft, and whether the navigation controls on board can be hacked in any way. The roadmap mentions evaluating privacy at the test sites as well but doesn’t specify how that testing will be conducted.

The roadmap does not apply to drones already being used by the U.S. government along large sections of the country’s border with Mexico.

Test site operators must have a publicly available privacy plan and abide by state and federal privacy laws. The plan must be reviewed annually, with opportunities for public comment.

"The FAA is also actively engaged in interagency efforts to develop privacy safeguards as (drones) are integrated into the national airspace," the statement said.

The FAA estimates that within five years of being granted widespread access, roughly 7,500 commercial drones, many of them smaller than a backpack, will be buzzing across U.S. skies.

So far there are applications to serve as test sites from 25 states, as consortiums involving industry and local governments compete fiercely to be selected as testing grounds.

The Teal Group, an industry forecaster in Fairfax, Va., estimates that worldwide annual spending on drone research, development, testing and evaluation (RDT&E) and procurement will increase from $6.6 billion in 2013 to $11.4 billion in 2022.

In light of revelations about spying by the National Security Agency (NSA), Calabrese says the public is worried.

“I think the biggest privacy concern is that we see drones in relation to what the NSA has done to the Internet," he said. "It’s mass collection of personal information on everybody in public, with little or no control over how it’s used or shared.”

Dexter Mullins contributed to this report. With The Associated Press.