FRANKFURT — Bidders for assets of insolvent German airline Air Berlin AB1.DE have until Sept. 15 to submit their offer, a spokesman for Air Berlin said in an e-mailed statement on Tuesday.

"Air Berlin will complete the investor process quickly," he added.

People familiar with the matter had told Reuters on Monday that the buyers of Air Berlin's assets would likely be picked by mid-September, as the race for the carrier's coveted take-off and landing slots in Germany heats up.

Lufthansa, Thomas Cook's Condor, easyJet and Ryanair are among airlines interested in the carrier's business or parts of it, sources familiar with the negotiations have said.

German aviation investor Hans Rudolf Woehrl is also working on a bid and former F1 driver Niki Lauda has indicated his interested in buying back Air Berlin's Niki, the Austrian airline he once owned.