UPDATE 2-US FAA considering stricter rules for high-volume charter flight operators

(Adds SkyWest shares, comment from former lawmaker, details on FAA legislation stalling)

By David Shepardson

WASHINGTON, Aug 24 (Reuters) - The Federal Aviation Administration (FAA) said on Thursday it is considering subjecting high-volume charter flight operators to stricter regulations imposed on passenger airlines.

Airline unions have opposed the expansion of charter operations. Association of Flight Attendants-CWA President Sara Nelson praised the FAA action, calling the charter regulations a "loophole that undermines safety and security rules for commercial aviation."

Charter flights, which can have a maximum of 30 seats, are not subject to the same training for first officers, mandatory retirement of pilots at age 65 or minimum rest periods for airline pilots. Passengers on those flights also need not be screened by the Transportation Security Administration.

The number of charter flights has grown dramatically over the last decade.

The FAA said on Thursday "this rapid growth poses an increased risk to safety if left unchecked" and said it will "begin a rulemaking to address this safety risk."

Utah-based regional carrier SkyWest Airlines operates regional flights for United Airlines, Delta Air Lines, American Airlines and Alaska Airlines and wants approval to operate a separate charter service called SkyWest Charter.

The Transportation Department has not yet acted on the request.

SkyWest Charter said it believes the charter designation "is essential for small community air service, today and well into the future." The air carrier added it "already exceeds current safety requirements." SkyWest Inc shares fell 2.4% Thursday to $40.28.

Aviation unions and American Airlines have criticized growing charter operations by air carrier JSX. American said in May that JSX had a "competitive advantage" by operating under less stringent regulatory rules.

JSX had 110,000 departures since 2016 with its 37 aircraft, the company said, while unions said that is more than some small regional passenger airlines, unions argued.

JSX said on Thursday the FAA notice "is disappointing, both for the traveling public and for the advancement of vital air mobility programs around the nation ... JSX complies with all applicable rules and regulations and far exceeds the safety requirements germane to our public charter operations."

Dallas-based JSX counts both United Airlines and JetBlue Airways as investors.

Former House Transportation Committee chair Peter DeFazio, a Democrat, praised the FAA for planning "to rein in JSX and others who are flying scheduled 'charters' to avoid pilot training requirements, proper rest and duty time (and) no retirement age."

Separately, a bill to extend the FAA's authorization for another four years has stalled in Congress over a debate about whether to revise pilot training requirements.

(Reporting by David Shepardson in Washington Editing by Matthew Lewis & Simon Cameron-Moore)