ATLANTA—The red ink is mounting for airlines amid soaring fuel costs, leaving them little choice but to further hike ticket prices. As Delta Air Lines and American Airlines reported big second-quarter losses Wednesday, they signaled customers should expect more hits to their checkbooks.

There are signs overall demand for flying within the U.S. is softening, but industry observers insisted that will not stop rising fares, more fees and fewer domestic flights.

“The first thing they have to do is forget about the butts in the seats and worry about the bucks in the till,” Minneapolis airline expert Terry Trippler said.

That point was underscored as Atlanta-based Delta Air Lines Inc. reported a $1.04 billion loss in the April-June quarter and Fort Worth, Texas-based AMR Corp., the parent of American, posted a $1.45 billion loss for the quarter. One-time charges and unprecedented fuel costs impacted both airlines, which saw their shares soar as their results beat Wall Street expectations and oil prices dropped.

The two carriers have posted combined net losses of $9.2 billion since the start of the year.