(CNN) -- Blaming the rising cost of traditional booking methods, Delta Air Lines is adding a $2 surcharge to all domestic roundtrip bookings except those made on its Web site.

Delta notified travel agents of the surcharge ($1 for each domestic segment of a trip) on Monday by posting a notice on computer reservations systems. The airline made no public announcement.

The surcharge applies to tickets bought from travel agencies as well as those purchased over the phone from Delta and at the airline's ticket counters and offices. It's also added to tickets purchased from Web sites other than the airline's own -- including sites such as Travelocity, Expedia, Preview Travel and the Internet Travel Network (which provides the functionality for CNN Interactive's Reservation Desk).

The American Society of Travel Agents is criticizing Delta for the move.

"They're blaming and punishing the traveling public for purchasing tickets through channels that (consumers) clearly prefer," said James Ashurst, an ASTA spokesman. "The Web still represents a very small amount of the overall tickets that are purchased."

In its announcement to travel agents, Delta said the surcharge will offset rising distribution costs -- including travel agent commissions, computer reservation system fees and credit card expenses.

For example, the airline said, the computer reservation system booking fees it pays have increased more than 280 percent since 1990, compared to Delta's revenue growth of more than 50 percent for the same period. The airline said three of the four major computer reservation system companies have announced increases from 2.9 percent to 5.2 percent for early 1999.

Delta said its Web site is its lowest cost distribution channel.

The airline is using the surcharge to push consumers toward booking on its Web site, where they won't have access to comparative information, Ashurst said.

"They're never going to tell you that United or TWA or some other airline has a cheaper fare for the exact same route," he said.

The ASTA spokesman said it's too early to tell how Delta's move will affect the business of travel agents. "As far as we know, no other carriers have matched this move," said Ashurst.