(Bloomberg) -- Less than a week after hitting a three-year high, European airline stocks are having their worst day in more than six months.

A UBS Group AG basket of eight European airline stocks fell as much as 4.2%, the steepest drop since Aug. 1. Sentiment was clouded by German tour operator TUI AG, whose shares fell 8% after it reported a slowdown in bookings for the crucial summer season. Rising prices for jet fuel also weighed.

The sector has rallied strongly over the past six months, led by an extension of gains in British Airways parent IAG SA and also helped by optimism over summer bookings from budget carriers EasyJet Plc and Ryanair Holdings Plc.

Yet the industry hasn’t been without setbacks. Some airlines have experienced delivery delays that shrank their passenger capacity. Hungary-based Wizz Air Holdings Plc cut its profit outlook last month, weighed down by engine issues that grounded some of its Airbus planes.

However, some analysts remain optimistic about the outlook for the sector. Europe’s airlines are still set to save on fuel costs this year, despite price increases that have been happening since September, Ruairi Cullinane, RBC Capital Markets analyst, said in an email.