U.S. Southwest Airlines has grounded dozens of planes following allegations that the airline broke federal safety rules, the airline said on March 12.

Forty-four Boeing planes were grounded "to determine whether they should go through further safety inspections," Southwest spokeswoman Whitney Eichinger told reporters.

But she did not confirm whether they were fuselage inspections or rudder inspections, issues which have recently raised concerns among Federal Aviation Administration (FAA) officials and congressional investigators.

Linda Rutherford, another Southwest spokeswoman, said the planes will be returned to service after inspection and the airline expects minimal disruptions for customers.

The announcement came a day after the airline said it was placing three employees on administrative leave.

The FAA has said it may levy a record fine of US$10.2 million against Southwest if it's found to have violated the federal aviation regulations.

It has submitted documents to congressional investigators alleging that the airline flew more than 100 planes in violation of mandatory safety checks.

The FAA said Southwest operated 46 Boeing 737s on nearly 60,000 flights between June 2006 and March 2007 while failing to comply with FAA directive requiring repeated inspections of fuselage areas to detect fatigue cracking.

The FAA also alleges that after Southwest discovered it had failed to comply, it continued to operate the same planes on an additional 1,451 flights in March 2007.

The airline later found that six of the 46 planes had fatigue cracks, the FAA said.