Facing significant hurdles, widespread drone access to U.S. skies will take longer than Congress had anticipated under a five-year roadmap for regulating unmanned aircraft released Thursday.

For the next several years, use of drones will be limited to permits granted case by case by the Federal Aviation Administration to operators who agree to procedures to reduce safety risks, the agency said.

Last year, Congress directed the FAA to grant drones access to U.S. skies by September 2015. But the agency already has missed several key milestones. The road map released Thursday had been due last February. Regulations providing for the safe integration of drones weighing less than 55 pounds (25 kilograms) were expected more than a year ago, but have repeatedly been delayed. The selection of six drone test sites around the country is also more than a year overdue.

FAA Administrator Michael Huerta said Thursday the agency's goal is to meet the 2015 deadline, but he appeared to narrow the definition of what it means to integrate drones into the national airspace.

"To me what that means is demonstrating what safe integration looks like, what its characteristics are, and to have a framework in place and some initial work actually ongoing in that area," he told a forum hosted by the Aerospace Industries Association, which represents many companies that make drones.

Among the concerns are whether remotely controlled drones will be able to detect and avoid other aircraft as well as planes with pilots on board. There are also security concerns, including whether drones' navigation controls can be hacked or disrupted.

"Government and industry face significant challenges as unmanned aircraft move into the aviation mainstream," Transportation Secretary Anthony Foxx said in a statement.

The road map has one big gap: privacy, one of the most widespread concerns associated with drones. It only addresses privacy related to the use of drones at the six test sites, which have not yet been selected. Test site operators must have a publicly available privacy plan that explains what data will be gathered and retained, and abide by state and federal privacy laws. The plan must be reviewed annually with opportunity for public comment.

Data gathered from testing at the six sites will feed into an interagency group looking at privacy and other drone-related issues that includes the FAA, the Homeland Security Department and the Justice Department, among other agencies, Huerta said.

Beyond that, the agency said, privacy isn't within its purview. "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.

FAA officials have long contended that, as a safety agency steeped in technology, they have little expertise on addressing broad public privacy worries.

Still, the FAA has taken "an important step" through privacy standards for the six test sites, said Christopher Calabrese, legislative counsel with the American Civil Liberties Union.

"However, it's crucial that as we move forward with drone use, those procedural protections are followed by concrete restrictions on how data from drones can be used and how long it can be stored," he said. "Congress must also weigh in on areas outside of the FAA's authority, such as use by law enforcement and the Department of Homeland Security, which have the ability to use drones for invasive surveillance that must be kept in check."

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.

Industry-local government consortiums around the country are competing fiercely to be selected for one of the test sites. The FAA has received 25 applications from groups in 26 states, Huerta said.

The Teal Group, an industry forecaster, estimates worldwide annual spending on drone research, development, testing, and evaluation procurement will increase from $6.6 billion in 2013 to $11.4 billion in 2022.

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