A government watchdog faulted the Federal Aviation Administration (FAA) for allowing Southwest Airlines Co to fly jets that had not undergone mandatory maintenance work, violating stipulated safety rules, the Wall Street Journal said.

The Office of Special Counsel said on Thursday that the company failed to complete certain required maintenance involving rivets around the window frames of 55 of its Boeing 737 aircraft, the paper said, citing the counsel's documents.

Southwest voluntarily disclosed the information to a senior FAA inspector who was also a supervisor in the local office charged with overseeing the airline, the Journal said.

But the inspector did not determine why the company had never completed the repair work or ensure that the necessary maintenance work was eventually completed, the paper added.

These issues have been previously addressed through comprehensive audits by the FAA ... our own audits, and work by outside consultants, a Southwest spokesperson told the paper.

The spokesperson told the paper that the airline officials are confident that we have identified and remedied the issues.

A second investigation has shown that some FAA managers continue their willingness to look the other way rather than enforce strict compliance with maintenance requirements and voluntary disclosure rules, the paper said, quoting Associate Special Counsel William Reukauf.

Southwest Airlines and the FAA could not be immediately reached for comment by Reuters outside regular U.S. business hours.

(Reporting by Amulya Nagaraj in Bangalore)