Three of the nation’s largest airlines Thursday reported net profits in the second quarter largely because they were reimbursed for airport security fees by the federal government. But executives warned that, while costs are coming down, passenger revenue and ticket prices remain weak, and more losses are likely.

Delta, Northwest and Continental reported combined net profits of $490 million. Excluding one-time items, though, the carriers lost a total of $421 million. The stock prices of all three airlines fell Thursday.

The carriers said the second quarter was made difficult by high fuel costs, the war in Iraq and fears of severe acute respiratory syndrome. They also face increasingly tough competition from low-cost carriers.

The government’s security-fee assistance program is scheduled to end Sept. 30, and as far as Blaylock & Partners airline analyst Ray Neidl is concerned, “There’s no profits, they’re still losing money.”