US Airways acknowledged some drop-off but said it wouldn't assume any particular reason for cancellations.

"Generally this is a slow time of year for us anyway and we see bookings fall off a bit," a spokesperson told CNN, adding that the airline has a "Peace of Mind" plan prepared if the U.S. goes to war or enters a code red on the homeland security scale. "Once [the plan] is in place then we can start tracking to see if and why people do cancel their flights."

Two airlines reported no declines. "It is business as usual," a Southwest Airlines spokesperson said. American Airlines said travel loads have been "normal and even up to some degree."

All but one of the airlines said they have plans for changes if war breaks out, but only two would offer details. United Airlines said it was "prepared to reduce capacity by 10 to 12 percent if necessary." American Airlines will reduce flying schedules if necessary, but the airline was taking a wait-and-see attitude.

"American's contingency plan includes a reduction in the flying schedule, but it's too early to say how much of a reduction will be required," it said. "Once we determine the extent of the military action, we can make knowledgeable decisions about better matching our schedules to the expected drop in domestic and international passenger demand."