The federal government will close down 173 small airport towers next month, according to an industry association.

The Federal Aviation Administration told control-tower operators on Tuesday that they would shut down towers at 173 small airports April 7, the American Association of Airport Executives (AAAE) told ABC News. Another 16 towers will close in September, the group said.

The FAA did not immediately respond to requests for comment.

The closures would affect small airports, many of them rural, where towers are manned by unionized contractors rather than FAA employees, according to AAAE, a trade association representing companies that operate those airports.

The FAA oversees 250 contract towers, according to a November report by the department's inspector general.

"Contract towers continue to provide air traffic control services at a lower cost than similar FAA towers. Based on our review of 30 randomly selected contract and 30 FAA towers with a comparable level of operations, a contract tower cost, on average, about $1.5 million less to operate than a similar FAA tower," Assistant Inspector General for Aviation and Special Program Audits Jeffrey Guzetti wrote in the report.

The FAA notified operators in a conference call on Tuesday, according to AAAE President Spencer Dickerson, who said the closures would be unprecedented.

"In the 30-year history of the contract tower program, three have shut down," Dickerson told ABC News. Most of the employees are former FAA and military air-traffic controllers, Dickerson said.