DALLAS (AP) — Get ready to spend more on travel. Airlines are raising ticket prices again after a long lull that coincided with falling fuel costs.

Over the weekend several big airlines matched United Airlines' increase in base fares of up to $10 per round trip within the U.S.

Fare watchers said it was virtually certain that the new prices would stick, resulting in the fourth fare increase this year and the first since late March.

United raised fares Friday by $4 to $10 per round trip, depending on flight length. Over the weekend Delta Air Lines, American Airlines, US Airways and JetBlue all matched that. So did Southwest Airlines and its AirTran subsidiary on flights of at least 500 miles. Southwest carries more passengers within the U.S. than anyone and is seen as a price-setter on many routes.

"Southwest participation virtually assures the hike will stick," said Rick Seaney, CEO of FareCompare.com.

The airlines raised domestic base fares nearly a dozen times last year, citing rising fuel prices. But they grew more cautious this spring as the economy weakened and fuel prices fell along with a decline of about 20 percent in oil prices.

Not all passengers pay the higher base fares. Airlines constantly promote various sales, blunting the impact of their base-fare increases.