Hoping to rebuild its capital base, the Federal Home Loan Bank of San Francisco said it will not pay a dividend for the first quarter and will not repurchase excess stock held by members on April 30.

The Home Loan bank said the moves stem from worries that it could face additional other-than-temporary impairment charges on its private-label mortgage portfolio that would further erode its capital. The bank recorded $590 million in such charges last year, while retained earnings fell 22.5%, to $176 million.

In a letter Friday to members, Dean Schultz, the bank's president and chief executive, said more OTTI charges could be in the offing.

"Ongoing stresses in the credit markets, substantial declines in real estate values, and weakness in the U.S. economy are continuing to affect the loan collateral underlying the bank's nonagency" mortgages, he wrote. "As a result, it is likely that the bank will incur credit losses on some of these securities at some future time and will record impairment charges."

Schultz said the bank is "currently evaluating" guidance issued this month by the Financial Accounting Standards Board that might ease the impact that OTTI charges have had on the industry.

Meanwhile, the Federal Home Loan Bank of Boston said Monday that it lost more money during 2008 than it originally anticipated thanks to $42.7 million in additional OTTI charges.

The Boston bank said in its annual report that it lost $115.8 million during the year. When it reported unaudited numbers in February, it said $339.1 million in OTTI charges created a $73.2 million net loss.

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Corrected April 16, 2009 at 10:56AM: An earlier version of this story overstated the amount of other-than-temporary impairment charges the Home Loan Bank of San Francisco recorded last year. It was $590 million, not $1.5 billion.