Frontier Airlines Holdings Inc, the regional carrier that sought bankruptcy protection last month, yesterday joined AMR Corp's American Airlines and Delta Air Lines in increasing charges for youngsters flying without an adult. The fees are added to the fares for unaccompanied children from ages 5 to 14, Bloomberg News reported.

The charges expand on airlines' efforts to find more revenue from sources other than ticket prices as an 86 percent increase in jet fuel over the past year widens industry losses. Carriers are tacking on higher charges for baggage, pets and booking a reservation by phone as US summer travel begins.

"New and increased fees announced this month will generate several hundred million dollars in incremental annual revenue," American, the world's largest airline, said last week in announcing bag-check and other charges.

Hundreds of thousands of children fly solo each year on U.S. airlines. American carried about 200,000 unaccompanied minors in 2007 among its 98 million passengers, spokesman Tim Smith said.

Smith said American doesn't break out any expenses associated with children traveling alone. Across the industry, carriers are raising fees for services they consider optional, such as overseeing youngsters flying by themselves.

American is raising its one-way charge for unaccompanied minors to 100 U.S. dollars from 75 U.S. dollars. Delta increased its fee to $100 each way for all trips.

American will become the first US airline to charge for a single checked bag, tacking on $15. The fee for oversized bags is increasing to $150 each way at American.