Some US airlines have shifted scheduled aircraft maintenance to contract facilities that receive little attention from government or airline safety inspectors, a government report said on Monday.

The Federal Aviation Administration has long allowed US carriers to complete minor tasks or bigger emergency jobs at lightly regulated repair sites in the United States and overseas to keep planes in the air on short notice.

But an audit of maintenance practices at 19 airlines by Transportation Department Inspector General Kenneth Mead found that six carriers, none of which was identified in the report, have pushed at least some scheduled maintenance, which can include engine, wing and fuselage inspections, to sites that lack direct FAA oversight.

Mead said "more significant work than anyone realized" is being performed at these locations and said the FAA had not been aware of it. Mechanics at these facilities are supposed to be FAA certified.

Under pressure to reduce costs to stem ongoing and substantial losses, US airlines contract about half of their maintenance to outside sources compared with 35 percent a decade ago. Much of the contracted work is performed at shops that are supposed to meet FAA quality control standards and are supposed to be regularly inspected, Mead said. But a growing amount of work -- about 40 percent -- is going to less regulated operations.

Some airlines find it convenient, faster and cheaper to send work to repair facilities that are not visited by agency inspectors.

The FAA did not contest most of Mead's findings and said it was up to airlines to monitor maintenance at these shops because its staffing was limited. The FAA will then check the quality of the airlines' oversight.

But Mead said neither the FAA nor the six carriers cited in his review provided adequate monitoring of scheduled maintenance performed at six US and overseas facilities. The airlines, Mead said, relied primarily on telephone follow-ups.

The review found no examples where safety was compromised.

Mead's report was requested by US Representative James Oberstar of Minnesota, the ranking Democrat on the House of Representatives Transportation Committee. The request was prompted by the deadly 2003 crash of an Air Midwest plane in North Carolina. Work at an outside repair facility was cited by investigators as a contributing cause of the accident.