U.S. aviation regulators proposed on Wednesday $9.2 million of combined safety-related fines against US Airways Group and UAL Corp's United Airlines , officials said.

The Federal Aviation Administration proposed a $5.4 million penalty against US Airways for operating eight planes while out of compliance with certain safety directives or its own maintenance programs.

The FAA proposed to fine United $3.8 million for operating a Boeing 737 on more than 200 flights after violating its own maintenance procedures on engine work.

Fines can be reduced on appeal, an option available to both airlines.

In United's case, the FAA said airline mechanics in April 2008 found two towels, instead of protective caps, covering an oil sump compartment inside one of the 737's two engines. The towels apparently had been in place since the previous December.

In the US Airways case, the FAA said it flew eight aircraft on 1,647 flights between October 2008 and January 2009 that were not in compliance with required safety inspections. Two of the directives required inspections of Airbus A320s for cracks on a landing gear part.

US Airways said in a statement it would work with the FAA to achieve a negotiated resolution of the agency's proposed penalty. The incidents cited by the FAA occurred during the carrier's integration of maintenance systems and it was working with the FAA to correct any problems, US Airways said.

(Reporting by John Crawley; Editing by Maureen Bavdek and Steve Orlofsky)