In a letter signed by David Cush, vice president and general sales manager, the airline noted that its full-content agreements with the GDSs expire this year, and it reiterated that it may not participate in all four systems.

Cush urged agencies to "consider adding other sources of fare content or leaving yourself the flexibility to add other sources of fare content."

American's move came in the wake of the agreement between Sabre and Amadeus that gives the GDS companies the option of backing each other up in the event an airline drops out of one of the systems. Kay Urban, chief executive officer of Amadeus North America, said the deal was struck to give travel agency subscribers confidence that they will continue to have access to airline content.

Most major U.S. carriers are currently negotiating new GDS agreements for the first time since the GDS industry was deregulated, allowing them to drop out of one or more systems if they choose. That, coupled with the emergence of alternative providers such as ITA Software and G2 SwitchWorks, has led to a certain amount of public posturing by the airlines.

The potential for fragmentation has generated some unease among agencies, which the Sabre-Amadeus agreement seeks to dispel.

The agreement does not apply to any airline that does not currently participate in both systems.

Chris Kroeger, senior vice president of Sabre Travel Network, North America, said Sabre "fully expects to have full content," but "we feel it is prudent to have a backup plan."

American also issued a statement that called the legality of the agreement into question.

"American believes that its contracts with GDS providers prohibit redistributing and remarketing American's content among those systems," it said.

Kroeger thinks otherwise. "I assure you that we looked thoroughly at our agreements with our airline customers, and these types of arrangements are allowable and valid."

American's suggestion to agencies that it might seek reimbursement if they book through expensive channels is reminiscent of Northwest Airlines' attempt in August 2004 to impose a "cost sharing" fee on agencies that booked the carrier through GDSs, rather than its dedicated agency Web site.

But at the time, Northwest was bound by its full-content agreements with the GDSs, whose lawyers argued that adding a fee to GDS bookings violated those contracts.

Northwest responded to the Sabre-Amadeus agreement with a terse statement: "We have one agreement covering Sabre subscribers and one agreement covering Amadeus subscribers and we expect the parties to respect those agreements." Northwest recently signed a new, five-year full-content agreement with Sabre that includes the settlement of litigation over its "cost-sharing" fee.