The insolvent German carrier Air Berlin canceled around 70 flights on Tuesday after many pilots called in sick.

"We are currently recording an unusually high number of sick notes among pilots," a company spokesperson said.

Air Berlin urged affected passengers on its website not to travel to airports and to instead contact its hotline.

German newspaper Bild reported that the cancellations were caused by "a revolt among pilots" over concerns that their jobs would not be saved after Air Berlin was sold.

Air Berlin is the country's second largest airline and employs around 8,600 people.

It filed for bankruptcy last month as its biggest investor Etihad Airways withdraw support after years of losses.

However, Air Berlin's starting and landing rights at Duesseldorf and Berlin airports, as well as its profitable subsidiary Fly Niki, are seen as highly-prized assets in the industry.

Parties interested in acquiring the carrier can enter the bidding process until Sept. 15. A formal decision on a sale of Air Berlin could be reached as early as Sept. 21.