The U.S. Federal Reserve's balance sheet expanded to a new record size in the latest week, even as the Fed shed some agency debt and mortgage-backed securities, Fed data released on Thursday showed.

The increase came as the central bank bought U.S. Treasuries as a part of its $600 billion program in an effort to help the economy.

The balance sheet -- a broad gauge of Fed lending to the financial system -- expanded to $2.516 trillion in the week ended February 23 from $2.492 trillion the prior week.