The U.S. Federal Reserve's balance sheet shrank slightly in the latest week on a small dip in its holdings of agency mortgage-backed securities, Fed data released on Thursday showed.

The Fed's balance sheet — a broad gauge of its lending to the financial system — slipped to $2.219 trillion in the week ended Dec. 30 from $2.221 trillion in the prior week.

The previous week had marked the highest balance-sheet liabilities total since the first week of 2009, when the financial crisis was still relatively fresh.

After declining early this year, the balance sheet generally has been accumulating mass amid the Fed's asset-buying program, in which the central bank's holdings of agency debt and mortgage-backed securities have grown to more than $1 trillion.

In the latest week, the Fed's holdings of agency mortgage-backed securities slipped to $908.26 billion from $910.43 billion in the previous week.

The Fed's holdings of agency debt remained steady at $159.88 billion.

Both of these figures are expected to resume their rise early next year, however.

By the end of March, the Fed plans to have bought $1.25 trillion worth of mortgage-backed securities and about $175 billion worth of agency debt.