The Federal Reserve is taking steps to boost the economic recovery by buying an additional $40 billion of mortgage debt each month.

The security purchases will start Friday and follow the Fed's "Operation Twist" program, where it sells short-term bonds to buy longer-term Treasury debt. The Fed also asserted it will extend its monetary policy by keeping interest rates at historic lows until at least mid-2015 from late 2014, a move punctuating concerns about the unemployment rate.

The latest purchases build on the $2.3 trillion in U.S. government and housing-related debt the Fed already has bought.

The economy created 96,000 jobs in August, well below the 250,000 jobs economists believe are needed to show healthy growth. The unemployment rate dropped to 8.1% from 8.3% but mainly because so many Americans found it futile to keep looking for work.

On Thursday, the Labor Department reported that weekly jobless claims rose by a higher-than-expected 15,000 to 382,000. While the department attributed about 9,000 of those seasonally adjusted new claims to Tropical Storm Isaac, the increase in claims showed a labor market heading in the wrong direction.

The Fed's move to buy more mortgage debt may be partly aimed at helping the housing sector, which has shown signs recently of renewed health. The pace of foreclosures has slowed, according to data released Thursday by RealtyTrac, which tracks home foreclosures and bank actions. But banks have turned to short sales, in which a lender agrees to accept less than the full mortgage balance when a home is sold, to avoid the expense of maintaining and then selling a foreclosed home.

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