Bond insurer Assured Guaranty, Hamilton, Bermuda, is facing uncertainty about mortgage and other structured finance exposures that it said it believes were too pessimistically assessed in recent downgrades.

"These assumptions are subject to considerable uncertainty that will dissipate within the next several months as the economy begins to benefit from the federal government's economic stimulus plans and mortgage assistance programs," Assured said, in response to Fitch Ratings' downgrades of its debt and insurer financial strength ratings.

However, the company acknowledged that the ultimate performance of its U.S. residential mortgage-backed securities is "uncertain."

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