Assured Guaranty Ltd., the Hamilton, Bermuda, bond insurer backed by the billionaire Wilbur Ross, has had a "breakthrough" in the past month in its efforts to get lenders to buy bad mortgages out of securities the company backs, said Dominic Frederico, its chief executive.

"Activity there has picked up, and we've started to achieve a result I'm incredibly pleased with," Mr. Frederico said on an earnings conference call Thursday. "If it continues at this level, it could have a potentially significant impact" on the company's already recognized losses and potential ones.

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