Low-fare Spirit Airlines Inc. is raising the stakes against financially weakened American Airlines with new service to Dallas-Fort Worth.

Spirit began flights Thursday from American's home hub at Dallas-Fort Worth International Airport to three U.S. destinations including Atlanta and New York's LaGuardia Airport. It will add three more next month including Boston and Phoenix, and it announced further expansion plans.

Spirit said it will add flights in May from DFW to Tampa, Fla., and Myrtle Beach, S.C., and has asked the federal government for permission to fly to Toluca, Mexico.

American and its short-haul affiliate, American Eagle, control about 85 percent of the flights at DFW. American, Eagle and parent AMR Corp., filed for bankruptcy protection in November.

Spirit chief marketing officer Barry Biffle said the airline sees an opening to undercut other DFW carriers on price. Spirit typically offers deeply discounted fares but adds many fees, most of them optional, covering everything from buying a ticket online to getting a drink of water during a flight.

American spokesman Tim Smith said the company had no comment on Spirit's plans.

Bob Mann, an aviation consultant in Port Washington, N.Y., said Spirit sees an opportunity at DFW but isn't the first — or most important — carrier to target a weakened American Airlines.

"More worrisome for American must be Delta's six-times-daily addition to New York," Mann said. Delta, the nation's second-largest airline, is building up its business at New York's LaGuardia after gaining more takeoff and landing slots there in a trade with US Airways.

DFW is one of American's five key U.S. hub airports. CEO Thomas Horton has said that American expects to shrink modestly while reorganizing, but he announced last week that the airline plans to grow by at those hubs by 20 percent over the next five years, partly with more international connections.

On Thursday, Spirit's shares fell 18 cents to close at $18.24.