The merger fever in the US airline business is showing no signs of abating.

Southwest Airlines, the biggest low-cost carrier in the United States, said in a surprise announcement that it had agreed to buy a smaller rival, AirTran Airways, for US$1.4 billion, reported the International Herald Tribune.

The planned merger would be the third in the past two years in the battered airline industry. Delta Air Lines bought Northwest in 2008, and a merger of United Airlines and Continental is due to be completed on Friday.