The company said holidaymakers were avoiding Turkey, usually a popular destination, because of safety fears.
Travel to Belgium had also seen a "sharp decline in demand" after the Brussels attacks.
Revenues for the past six months fell to £2.67bn, down from £2.74bn for the same period last year.
The company said the Brussels airport attack in March had significantly affected its visitor revenues from Belgium, due to "operational disruption to our flying programme, a high level of cancellations and a significant drop in customer demand".
But pre-tax losses for the six months to 31 March fell slightly from £303m to £288m because the company said it was selling higher quality holidays and improving margins.
And chief executive Peter Fankhauser said the picture in other parts of the business looked slightly rosier.
"Thomas Cook is trading well to destinations other than Turkey, with particularly strong bookings to Spain and the USA."
He added that the company was "well positioned to meet our existing growth expectations".
Thomas Cook said it anticipated that underlying operating profits for the year would fall between £310m and £335m, at the bottom of the range forecasted by analysts.
Thomas Cook's share price has fallen 19% after the travel company announced summer bookings were down 5% compared with last year.