The amount of soured U.S. housing debt backing the securities that roiled the global financial system declined last month for the first time in almost four years, according to Amherst Securities Group LP.

Mortgages at least 60 days delinquent in so-called nonagency bonds without government-backed guarantees, or "re-performing" after reaching that status, fell 0.3%, to $608.6 billion, according to a report Wednesday by the Austin securities firm.

The first decrease since at least June 2006 was caused by increased liquidations of properties after borrowers stopped paying and a further drop in new defaults to 1.3%, partly reflecting "seasonality" and the lowest reading since 2007, according to the firm.

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