INDIANAPOLIS ( TheStreet) -- Simon Property ( SPG) is readying a new takeover bid for bankrupt rival General Growth Properties that would top offers from several General Growth investors, the Wall Street Journal reports, citing a letter sent by Simon to General Growth's lawyers. Simon didn't outline the details of its offer, but said it expects to deliver its improved proposal late this week or early next, people familiar with the matter told the newspaper. Simon also assured General Growth of resolving antitrust concerns arising from the deal. Last month, Simon offered to buy General Growth for more than $10 billion. Simon is working in tandem with private-equity company Blackstone ( BX) and two unidentified sovereign-wealth funds to come up with a better offer. In addition, it also is lining up a $6 billion credit line led by JPMorgan Chase , Citigroup and Morgan Stanley among others to help finance the bid, Simon's letter said. When asked about the letter, Simon Chairman and CEO David Simon told the Journal, "We've made no final decision on what we're going to do." Simon's offer would need to top a recapitalization proposal unveiled earlier this month by Brookfield Asset Management ( BAM) and General Growth investors Fairholme Capital Management and Pershing Square Capital, which would split General Growth into two entities upon emerging from bankruptcy valuing it at $15 a share. Follow TheStreet.com on Twitter and become a fan on Facebook.