The chief executive of Simon Property Group Inc on Thursday said a plan by management of General Growth Properties Inc to emerge from bankruptcy lacks certainty despite new financial backing.

Simon, the largest U.S. mall owner, last month offered to buy General Growth, the second-largest, for $10 billion in a plan that had support from many General Growth unsecured creditors.

But General Growth on Monday set forth a competing plan to emerge from bankruptcy, under which William Ackman's Pershing Square Capital Management and fund manager Fairholme Capital Management would invest up to $3.93 billion.

That plan has backing from Canada's Brookfield Asset Management Inc . Pershing Square is General Growth's largest shareholder, and Fairholme its largest debtholder.

General Growth had earlier proposed to split in two companies, in a plan under which Brookfield would have invested roughly $2.63 billion in exchange for a 30 percent stake.

That plan also called for General Growth to sell about $1 billion of assets and raise $3.3 billion of equity.

David Simon, the chief executive of Simon Property, on Thursday said the revised plan retains potential pitfalls for General Growth shareholders.

The first plan had a lot of uncertainty to it, in terms of how it was going to impact shareholders, Simon said at a New York University conference. Their added plan eliminates some uncertainty, but not all of it.

The case is In re: General Growth Properties Inc, U.S. Bankruptcy Court, Southern District of New York, No. 09-11977.

(Reporting by Paritosh Bansal, Ilaina Jonas and Jonathan Stempel, editing by Dave Zimmerman)

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