Things are starting to get ugly in the battle between airlines and travel-booking websites. Less than one week after American Airlines pulled its listings from Orbitz.com, Delta has announced its flights will no longer be listed on three other sites.

The sites — CheapOAir.com, OneTravel.com, and BookIt.com — are small when compared to the user base of Orbitz, but could represent the thin edge of the wedge as airlines aim to push customers to booking directly on their sites and away from third-party sites that receive commissions from the airlines for each fare.

There has been some push back from the booking sites. After American bailed on Orbitz, fellow travel site Expedia.com responded by burying American’s listings on the site.

“American Airlines has shown it only intends to do business with travel agencies through a new model that is anti-consumer and anti-choice,” wrote Expedia about its decision.

In recent months, Southwest Airlines has been using TV ads to hype the fact that consumers can only buy Southwest tickets online through the carrier’s website.

The war in the air: Delta joins American as latest airline to drop travel booking sites [NY Daily News]

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