A recent uptick in flight delays at major U.S. airports may signal the start of summer-long travel disruptions, a government watchdog said.

The warning from the Transportation Department (search) Inspector General's office came as millions of travelers took advantage of low fares and plentiful flights for trips over the long Fourth of July holiday.

U.S. airlines expect to fly more than 200 million passengers from the end of May through the first week in September, a 4 percent increase over last year.

Following a drop in April and May, delays rose sharply at key airports during the first two weeks of the summports had delays affecting at least 25 percent of flights.

The rate at nine airports — including Newark, Philadelphia, Miami, Washington Dulles and Chicago O'Hare — was 30 percent or greater. At O'Hare (search), the government has brokered an agreement among major carriers to limit flights through October.

Airline delays are usually worse in summer because of thunderstorms. Federal Aviation Administration forecasters are predicting a heavy storm season and storms were bad in the Midwest in early June.

The worst airport for delays was Atlanta Hartsfield (search), where 35 percent of flights were delayed during the first two weeks of June.

A commercial flight that is 15 minutes late is considered delayed. Ten airports had delays that averaged more than one hour and the worst was New York's LaGuardia with an average wait of 71 minutes.

"It's too soon to tell whether this is a short-term condition resulting from anomalous weather or if it is a more pervasive problem," Mead's report said.

But Mead's office said regulators should be watching all congested airports closely to determine if broader federal intervention is necessary.

The surge in delays is partly driven by low-cost carrier growth in bigger markets, hub consolidations by the biggest airlines, increased regional jet traffic and a surge in passenger growth, Mead said.

U.S. airlines, trying save money, plan to fly planes full this summer and wait out delays rather than cancel flights, the FAA has said.

In a departure from previous practice, airlines are not expected to keep significant backup aircraft on hand to respond to big summer delays. Carriers want to manage setbacks and stretch out traffic throughout the day.

Delays cost airlines more than $6.2 billion in operating expenses in 2004, industry figures show.