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STORY: Shares of Delta Air Lines fell Thursday after the company forecast lower-than-expected profit for the third quarter.

The airline cited discounting pressure.

Delta and other carriers have added more seats amid a summer travel boom, forcing them to discount fares.

Rivals American Airlines and Southwest cut their revenue forecasts in the second quarter, also citing discounting pressure.

The average round-trip ticket for a U.S. domestic flight was $543 in May, down 1% month-over-month and 3% lower from a year earlier, according to data from Airlines Reporting Corporation.

Most of Delta's shortfall comes from its main cabins, which generate about 49% of passenger revenue.

That revenue was flat in the June quarter, while revenue from its premium cabins was up 10% year-over-year.

The company said its pricing power will "significantly" improve beginning in August as U.S. carriers start to slow seat additions in the U.S.

Shares of United and American were also down in Thursday trading.