Ambac Financial ( ABK) and other bond insurers are working on a plan to send to the Treasury Department that would enable them to sell troubled assets to the government, Bloomberg reports. The companies also may present a proposal next week that would allow the insurers to guarantee some assets with government backing, CEO Michael Callen said in a Bloomberg interview. ``We're working hard to put together a proposal and it's got to be an industry proposal,'' Callen said, Bloomberg reports. ``We don't have a lot of time.'' Callen said he isn't asking Treasury to take a stake in Ambac. The U.S. Treasury said Thursday it isn't considering buying equity holdings in bond insurers, Bloomberg reports. Five of seven bond insurers, including MBIA ( MBI) and Ambac, lost their top AAA ratings this year as losses surged on securities linked to subprime mortgages, reports Bloomberg.