DETROIT - General Motors Co Chief Executive Ed Whitacre said on Wednesday he is not hopeful a breakthrough deal can be reached to save Saab and said GM is proceeding with the wind-down of the Swedish brand.

Whitacre, speaking to reporters at the company's headquarters, said none of the potential bidders for Saab have come forward with the financing needed to restructure the money-losing automaker.

I think we've done everything humanly possible, Whitacre said.

GM will start closing down Saab plants later this week, he said.

Dutch luxury carmaker Spyker Cars is among possible bidders for Saab. Spyker Chief Executive Victor Muller said last week GM had set a Thursday deadline for bids.

Separately, Whitacre said the No.1 U.S. automaker aims to close a deal to sell the Hummer SUV brand to China's Tengzhong Heavy Industrial Machinery by the end of January. GM reached a tentative deal with Tengzhong to sell Hummer in June.

(Reporting by Soyoung Kim; Editing by Tim Dobbyn)