WASHINGTON — There are too few fully qualified controllers at more than a dozen of the nation’s busiest air traffic facilities stretching from Atlanta to Anchorage, according to a report released Tuesday by a government watchdog.

The 13 airport towers, approach control facilities and en route centers have fewer fully trained controllers than the minimum number established by the Federal Aviation Administration specifically for each facility, Transportation Department’s inspector general said.

The FAA considers the facilities fully staffed because controllers still in training are used to fill the gaps. But the report says there is great variation among trainee skill levels and readiness to work on their own.

It typically takes about three to five years for a trainee to become fully qualified. Many trainees need fully qualified controllers to sit alongside and watch while they direct air traffic, ready to step in if there is a problem. Other trainees are able to work alone.

The report also questions the validity of the minimum staffing levels the FAA has assigned to the facilities, finding fault with the agency’s methodology.

The report comes as members of Congress gear up for a fight over whether to spin off air traffic control operations from the FAA and place them under the control of a nonprofit corporation made up of airlines, airports and other aviation “stakeholders.” Rep. Bill Shuster, R-Pa., chairman of the House Transportation and Infrastructure Committee, is expected to introduce a bill within the next few weeks.

The concept has the support of most of the airline industry with the exception of Delta Air Lines. But key House and Senate Democrats, as well as some business and general aviation groups, are opposed.

The inspector general’s office recently said in a separate report that spending on air traffic control operations has doubled over two decades, while productivity has declined substantially and efforts to improve performance have been ineffective.

Managers at some the 23 key facilities examined in Tuesday’s report cited a higher number of controllers needed to fill all work shifts than the FAA’s designated minimum number of personnel for that facility.