HOUSTON, July 29 -- The U.S. Federal Aviation Administration (FAA) said on Tuesday that it plans to fine Southwest Airlines 12 million U.S. dollars for failing to comply with FAA requirements on repairs and continuing to fly improperly-repaired planes.

The FAA said in a statement that from 2006, the Dallas-based carrier conducted so-called "extreme makeover" alterations to eliminate potential cracking of the aluminum skin on 44 jetliners, some of them Boeing 737s.

FAA investigators found one of Southwest's contractors, Aviation Technical Services Inc. of Everett, Washington, failed to follow proper procedures for replacing the fuselage skins on these aircraft.

"This could have resulted in gaps between the skin and the surface to which it was being mounted. Such gaps could allow moisture to penetrate the skin and lead to corrosion," the agency added.

They also determined that the contractor failed to follow required procedures for placing the airplanes on jacks and stabilizing them.

The FAA said Southwest returned the airplanes to service in 2009 and kept flying them even after the FAA became aware of the unapproved methods of work and before the FAA had approved the repairs.

"Safety is our top priority, and that means holding airlines responsible for the repairs their contractors undertake," U.S. Transportation Secretary Anthony Foxx said in the statement.

In response, the airline said in a press release that it had "fully resolved the repair issues some time ago," and assured that "none of the items raised in the FAA letter affect aircraft currently being operated by Southwest Airlines."

"Safety is paramount and we always strive for full compliance with established and approved processes and procedures," the release said.

According to U.S. media reports, it is the second-largest fine the FAA has ever imposed an airline. The largest was against American Airlines and three of its subsidiaries, a fine of 24.9 million dollars in May 2013 related to the forced grounding of the airline's MD-80 fleet in 2008 when inspections found some wiring modifications had been improperly done.