STOCKHOLM - Luxembourg-based Genii Capital is working to improve its offer for ailing Swedish carmaker Saab, a spokesman for the investment firm said on Tuesday.

Genii is working on various improvements in its bid, Lars Carlstrom, who is coordinating a joint bid for Saab from Genii Capital and Formula One mogul Bernie Ecclestone, told Reuters.

Genii hopes to improve its bid so that it becomes more attractive for GM ... one has to sweeten it to be a part of the bidding, that's the message we've received (from GM).

Genii and Ecclestone submitted an 11th-hour bid for Saab last Thursday, joining Dutch luxury carmaker Spyker and a group of Swedish investors in trying to convince owner General Motors to sell instead of shutting it down.

GM executives said on Monday none of the bids received so far had met the financial requirement for a deal and signaled they were moving ahead with plans to wind Saab down.

Still, GM Vice Chairman Bob Lutz said the company was still open to offers from potential buyers, even though time was very limited.

Carlstrom said Genii planned to contact GM advisor Deutsche Bank on Tuesday and would make a statement during the day.

(Reporting by Jens Hansegard and Nick Vinocur; Editing by David Holmes)