WASHINGTON >> Drone flights over 133 military facilities will be restricted for security reasons, marking the first time such flight restrictions have been applied only to drones rather than aircraft in general, the U.S. aviation safety authority said today.

The restrictions are in response to requests from the Defense Department and other security and intelligence agencies, the Federal Aviation Administration said in a statement. No specific threat was cited.

While drones can’t fly over the facilities, the FAA’s agreement with the Defense Department permits drone flights below 400 feet high within the side boundaries of the facilities, the statement said. Small drones in general are limited by the FAA to flights no higher than 400 feet anywhere without special permission.

An attorney for a coalition of companies that want to make or use drones said the language of the restriction is broadly written and may be used to apply to other types of facilities besides military installations.

The FAA announcement came as a surprise to the drone industry, said Michael Drobac of the Small UAV Coalition. Companies in general weren’t asked for their views ahead of the decision despite efforts to work closely with the agency through numerous committees and working groups, he said. The “imprecise” language in the new restrictions leaves drone operators uncertain exactly what will and what won’t be allowed, he said.

“We lack the kind of clarity and specificity that industry depends upon from government,” Drobac said.

An aviation law passed by Congress last year requires the FAA to come up with rules to determine what types of facilities, in addition to military facilities, are safety or security critical and may be designated as no-drone zones. The agency was supposed to propose a plan to do that in January, but missed the deadline, Drobrac said.

FAA officials also promised to propose by the end of 2016 long-sought rules under which operators can fly drones over densely populated areas and crowds, which isn’t permitted now except by special waiver. The rules are a necessary step to clear the way for package deliveries by drones. But that proposal was derailed at the last minute when military and law enforcement officials expressed concern that they don’t have a way yet in which to identify whether drones operating present a threat or not.

FAA officials have now set aside their work on rules for flights over populated areas in order to first address the security concerns.

The new flight restrictions are effective beginning April 14. Only a few exceptions will be permitted and they must be coordinated with the individual facility or the FAA. Operators who violate the restrictions could be subject to fines or criminal charges.