DALLAS >> United Airlines said Monday that a key fourth-quarter revenue figure would be weaker than previously expected partly because of the effect on travel from the terror attacks in Paris.

United didn’t put a dollar figure on the effect. But it said that revenue for every seat flown one mile — a key figure in the business — was between 5.75 and 6.25 percent lower than in the same quarter of 2014. United had earlier expected a drop between 4 and 6 percent.

Besides the Nov. 13 Paris attacks, United Continental Holdings Inc. said it was affected by cutbacks in flying by energy-sector employees — the airline has a hub in Houston — and other factors.

The report came after United shares closed at $51.62, down 27 cents.

Also Monday, Southwest Airlines Co. said that traffic rose 8.6 percent in December, leaving fewer empty middle seats.

The average flight was 83 percent full last month, compared with 82.7 percent in December 2014.

Dallas-based Southwest said it continues to expect revenue for every seat flown one mile was flat to down 1 percent in the fourth quarter. That suggests average fares were lower than in late 2014.

Virgin America Inc., based in Burlingame, California, said that its December traffic rose 3.6 percent but occupancy slipped to 82.2 percent from 82.3 percent a year earlier.

Last week, Delta Air Lines Inc. reported that December traffic rose 1.6 percent and the average flight was 84.4 percent full, up from 83.7 percent the previous December. The revenue per seat figure fell 5 percent, with most of the drop attributed to currency-exchange rates and more Thanksgiving travel pushed into November.

Southwest shares fell 17 cents to close at $42.04; Virgin America shares fell 61 cents to $32.26.