JetBlue Airways Corp, which last week began offering a one-month unlimited travel pass, ended the sale early when the passes sold out before the promotion was set to expire, the airline said on Thursday.

The low-cost airline was offering $599 passes that allow holders to travel to any of JetBlue's 56 destinations. The passes are valid between September 8 and October 8. The offer was set to expire on Friday.

We wanted to ensure those who bought the pass could get the flights they want, so we capped the number of passes we would sell, said JetBlue spokeswoman Jenny Dervin. And we reached that number yesterday.

U.S. airlines, battered this year by an economic recession that has eroded travel demand, aim to bolster bookings in the fall with sales and creative marketing.

The Air Transport Association (ATA), an industry trade group, this week predicted the number of passengers on U.S. airlines during the Labor Day holiday would fall 3.5 percent.

Planes are likely to be full, however, because of sweeping capacity cuts last year and this year at major airlines.

Shares of JetBlue were up 4 cents to $5.12 on Nasdaq in early trade.