A group of US lawmakers is challenging airlines to justify rising fees for checking bags and making changes to flights, CNBC reports.

Senators Ed Markey and Richard Blumenthal, and Representative Steve Cohen asked 11 airlines to provide an estimate of the true cost of baggage handling and whether the administrative cost of changing flights had increased over the past decade.

The lawmakers noted that travellers often pay a low initial airfare only to be faced with added service fees later.

The airline industry earned about $4.6 billion on checked-bag fees alone last year, an increase of about one-third since 2012. Unlike airfares, the fees are not subject to a 7.3 per cent federal tax.

The trade group Airlines for America defended the practice. “To the extent that fees or other surcharges help airlines generate more revenues or avoid costs for services that some passengers may not value, they help overcome an economic climate in which costs are rising steadily, as they are today,” said spokesperson Alison McAfee.

US airlines first started charging travellers for checked bags about a decade ago. Alaska Airlines, JetBlue, and United Airlines recently raised the cost of checking a first bag from $25 to $30.