The U.S. Transportation Department has fined a Florida-based airline for advertising US$9 airfares for its flights from Los Angeles without clearly disclosing the full price, including taxes and fees.

Spirit Airlines, which promotes itself as an ultra-low-cost carrier, was fined US$50,000 Monday for violating federal aviation laws and Transportation Department rules that prohibit deceptive price advertising.

"We expect airlines to treat their passengers fairly, and we will take enforcement action when they violate our price advertising rules," U.S. Transportation Secretary Ray LaHood said.

For a short time in June, Spirit launched an advertising campaign to promote Spirit's five daily flights from Los Angeles to Las Vegas. The US$9 fares were advertised on billboards on the back of trucks and on hand-held posters carried by workers throughout Los Angeles, according to an investigation by the federal agency.

An asterisk and small print on the billboards said additional taxes and fees would apply but did not clearly disclose the costs, as required under federal rules. The hand-held posters, carried by workers trained to answer questions about the fares, did not include any information about extra fees and taxes, according to the Transportation Department.

David Stempler, an aviation attorney and president of the Air Travelers Assn., a nonprofit advocacy group based in Maryland, said passengers are frustrated with airlines that fail to clearly disclose their full fares. "When they don't include the full fare it makes it hard to compare prices," he said.

Spirit also publicized the US$9 one-way fares on Twitter but required that Twitter followers click on two separate links to read about the additional fees and taxes, according to the federal agency. A spokeswoman for the airline declined to comment on the fine.

A report by the Transportation Department said Spirit Airline officials responded to the fine by claiming that the omission of the taxes and fees was inadvertent and that the campaign lasted only two days in a few locations in Los Angeles.

Without admitting or denying wrongdoing, Spirit has agreed to pay the fine and to stop promoting such fares without clearly disclosing the extra taxes and fees.

With extra taxes and fees, the US$9 fares promoted by Spirit for flights between Los Angeles and Las Vegas are actually about US$35 each way, according to information provided on the airline's website. But to qualify for the US$9 base fare, passengers must join the airline's "US$9 fare club" for an annual fee of US$59.95.

Spirit describes itself as the "largest ultra low cost carrier" in the United States, Latin America and the Caribbean. It also promotes itself as "the unbundling leader" because it has an extensive list of extra fees for goods and services that are usually included or "bundled" into the ticket price. It is the only airline in the U.S. to charge passengers as much as US$40 to pack a carry-on bag into the overhead compartment of a plane.