German investor Hans Rudolf Woehrl has unveiled a 500-million-euro ($600 million) offer for bankrupt Air Berlin, hoping to buy the country's second-largest airline whole rather than see it split up.

"We worked hard to be able to make this offer, with help not only from experts but also supportive comments and suggestions from employees, passengers and business partners of Air Berlin," the Bavarian airline tycoon wrote in a Facebook post late Sunday.

Nuremberg-based Woehrl first rose to prominence when he bought airline Deutsche BA from British Airways for a symbolic one euro in 2003, later selling it to Air Berlin.

On Sunday his company Intro offered 50 million euros immediately for the stricken airline, with up to 450 million euros of further payments "depending on performance", he said.

Intro wants "Air Berlin as a whole" rather than buying up chunks, Woehrl emphasised, urging other potential buyers nosing around the airline like Lufthansa, Condor, TUI, Germania and Austrian former Formula One driver Niki Lauda to join his offer.

But he added that the firm could if necessary buy up all of Air Berlin's assets, including 140 leased aircraft and prized landing and takeoff slots at German airports.

The German government has argued that competition rules prevent any single airline taking over Air Berlin, the country's second-largest carrier after Lufthansa.

After losing the lifeline of regular cash infusions from Gulf carrier Etihad in June, Air Berlin filed for insolvency on August 15.

The firm has given potential buyers until September 15 to make offers.

In the meantime, the airline has been kept aloft by a hastily-agreed 150-million-euro loan from the German government.