By David Shepardson

WASHINGTON (Reuters) -A bipartisan group of U.S. senators harshly criticized rising airline fees for luggage and seat assignments on Wednesday, calling the fees unfair and citing how different customers are charged differently.

"Airlines these days view their customers as little more than walking piggy banks to be shaken down for every possible dime," Senator Richard Blumenthal, a Democrat who chairs the Permanent Subcommittee on Investigations, said at the hearing.

Executives of American Airlines, United Airlines, Delta Air Lines, Spirit Airlines and Frontier testified at the two-hour hearing.

Carriers are working on customer-specific pricing "to discriminate against passengers, and to raise fares and fees for consumers the airline believes will pay more," Blumenthal said in the hearing.

Republican Senator Josh Hawley questioned why some airlines charge passengers different fees for baggage on the same flight.

"This is Russian roulette," Hawley said. "Nobody enjoys flying on your airlines. It's a disaster. ... It's absolutely terrible"

A report released by Blumenthal last week disclosed that the five airlines collectively earned $12.4 billion in revenue from seat fees between 2018 and 2023.

Airlines say the fees, for both checked luggage and carry-on bags and seat assignments, are about customer choice, saying they let customers pay for what they need. The airlines do acknowledge that the fees are a key part of their revenue structure as they face rising costs.

A year-long investigation by Blumenthal's panel found that carriers are increasingly using algorithms to set fees. Blumenthal said he plans two additional aviation reports before the end of the year.

"The fury and frustration on both sides of the aisle was pretty dramatic," Blumenthal told Reuters after the hearing.

Lawmakers want the U.S. Department of Transportation to investigate airline practices and quickly finalize rules that would bar airlines from charging family seating fees. Some senators also called on airlines to drop their lawsuit seeking to block new fee disclosure rules.

Senators also criticized the airlines for a lack of transparency in how fees are disclosed to customers when they book a flight, something that the airlines deny.

“You charge people fees they know nothing about,” Hawley said. “You harass them to death.”

American Airlines Vice Chair Stephen Johnson said carriers need to "appeal to the most budget-conscious customers." Delta executive Peter Carter said the carrier provides "options and value for every customer. ... Fee practices that erode the trust and loyalty of our customers are not in our best interests."