The balance sheet of the U.S. Federal Reserve grew over the last week as the central bank continued with a plan to adjust its portfolio and stimulate an economy it expects to grow only modestly this year.

The Fed's asset holdings in the week ended March 7 were $2.896 trillion, up from $2.887 trillion a week earlier, it said in a weekly report released Thursday.

The Fed's holdings of U.S. Treasury securities grew to $1.660 trillion, up slightly from $1.659 trillion a week earlier. The central bank's holdings of mortgage-backed securities increased to $853.89 billion, up from $840.80 billion in the previous week.

The Fed's portfolio has more than doubled since the financial crisis of 2008 and 2009, as the central bank bought mortgage-backed securities and government bonds to keep interest rates low and stimulate the economy.

Fed officials plan to continue with a shakeup announced in September of the central bank's securities portfolio to hold more long-term government debt and mortgage bonds, a move intended to spur spending and investment by making borrowing cheaper.

Thursday's report showed total borrowing from the Fed's discount lending window was $7.38 billion on Wednesday, down from $7.43 billion a week earlier.

Commercial banks borrowed $3 million from the discount window. They borrowed $12 million in the previous week.

U.S. government securities held in custody on behalf of foreign official accounts grew to $3.473 trillion, up from $3.460 trillion the previous week.

U.S. Treasurys held in custody on behalf of foreign official accounts rose to $2.734 trillion, up from $2.720 trillion in the previous week. Holdings of federal agency securities edged down to $ 739.15 billion, down from $739.21 billion in the prior week.

Further data on the Fed's balance sheet, including a breakdown of district-by-district discount window borrowing, can be found at http://federalreserve.gov/releases/h41/current/h41.pdf

-By Kristina Peterson, Dow Jones Newswires; 347-882-7215; kristina.peterson@dowjones.com