DALLAS — The big U.S. airlines are raising base fares by $10 per round trip on some domestic routes with the busy summer travel season just around the corner.

United, Continental, Delta and American raised prices, but some of the low-cost carriers had not matched the increases by Friday afternoon.

The airlines face sharply higher costs for fuel, one of their biggest expenses. They have pushed through at least six broad price increases this year, but the last two in March failed when some airlines declined to go along.

"It's the good old college try — if it doesn't work, you try again," said Tom Parsons, CEO of travel website BestFares.com. "One week it's going to stick."

United and Continental raised prices Thursday night, said spokeswoman Julie King. Both are owned by United Continental Holdings Inc., the nation's largest airline operator. They were followed Friday by Delta and American and in part by US Airways, according to airline officials and airfare trackers.

Southwest, AirTran Airways and JetBlue said they had not matched the increase.