The Federal Reserve has auctioned nearly $25 billion in super-safe Treasury securities to big investment firms, part of an ongoing effort to relieve credit strains.

The auction - the fourth of its kind - was held Thursday, where the Fed auctioned another $24.999 billion in the securities. Bidders paid an interest rate of 0.1%. The Fed received bids of $35.1 billion worth of the securities.

In exchange for the 28-day loan of Treasury securities, bidding firms can put up more risky investments, including certain shunned mortgage-backed securities, as collateral.

In the four auctions held so far, the Fed has provided close to $158.95 billion worth of the Treasury securities to financial firms.