Tourist numbers in the country have plummeted as a result of terrorist attacks and political upheaval.
Bookings were up by eight per cent when Turkey was excluded, driven by demand for "high-volume destinations including the Balearic and Canary Islands and the USA, alongside smaller destinations like Bulgaria and Cuba" - but taking Turkey into account, bookings fell by four per cent.
A nine per cent decline in bookings for continental Europe was put down to weaker consumer confidence - due in part to a significant reduction in demand for Belgium, as a result of the Brussels terror attacks.
Northern Europe bookings dropped by six per cent, which the firm said was in line with capacity cuts.
Meanwhile, sales for the winter season are "in line with last year", the group said.
The company's share price dipped by 2.36 per cent in mid-morning trading.
"Sales so far for summer 2017 suggest that customers are booking early in an effort to secure their first-choice destination and hotel," said Thomas Cook chief executive Peter Fankhauser.
"We remain focused on ensuring that we have the right holidays available in the most popular destinations in order to meet changes in customer demand. At the same time, we continue to transform our business for profitable growth.
"We're particularly proud of the improvements we've delivered in customer satisfaction thanks to the work we've done to strengthen the quality of our offering. We know that the increased loyalty we get from happier customers is key to driving the future success of our business."
Fankhauser said the company had taken "big steps forward" with its growth plans, including entering a new hotel sourcing partnership with Webjet, and launching Thomas Cook China.
"It’s been a difficult year for Thomas Cook with political unrest in Turkey and the threat of terrorist activity deterring tourists from travelling to some of its most popular destination," said Hargreaves Lansdown senior analyst Laith Khalaf.
"The good news is things haven’t got worse since the beginning of the summer, but the weaker demand for Turkey as a destination continues to weigh heavily on the tour operator’s German airline business in particular.
"Meanwhile there appears to be no abatement of the geopolitical tensions that have made 2016 such a troubling time for the travel industry."