Several airlines have raised base fares this week, and the drive to boost prices gained a valuable ally in Southwest.

The increases are $10 per round trip and apply to most routes in the U.S., according to fare experts.

Airlines have raised prices several times since December, saying they need to cover the rising cost of jet fuel. American Airlines parent AMR Corp. said Wednesday that due to high prices it expects to spend $1.2 billion more on fuel this year than last year.

FareCompare.com CEO Rick Seaney said the latest fare increase was virtually guaranteed to stick once Southwest and US Airways matched it. The last three efforts by other airlines to raise prices faltered, in some cases because Southwest declined to go along.

JPMorgan analyst Jamie Baker said Southwest may have wanted to pause to see if earlier fare increases would hurt travel demand.

Southwest spokesman Chris Mainz said the airline raised prices across its route network "to help offset higher fuel costs that continue to face the industry. It's all about fuel."

Baker and Seaney said Delta started the $10 fare increase on Monday and was followed by United, Continental, American, JetBlue, AirTran, Alaska, Frontier and Sun Country on the bulk of their domestic flights.

While airlines have raised base prices several times this year, they have offset some of the increase by offering fare sales.