WASHINGTON (REUTERS) - The Federal Reserve pumped US$77.7 billion (S$99 billion) into the U.S. Treasury last year, returning some interest payments made by the government on debt securities held by the central bank.

The figure published on Friday covers the central bank's earnings minus operating costs and other expenses, which the Fed sends to the Treasury every year.

The central bank's profits derived from US$90.4 billion in interest income on the securities in its multi-trillion-dollar portfolio of US government debt and bonds related to the housing industry.

The Fed's balance sheet ballooned over the last four years to nearly US$4 trillion as it bought debt securities to lower interest rates and spark a faster economic recovery. Just over half of the Fed's securities holdings are US Treasury debt.