WASHINGTON — The Federal Reserve announced Wednesday it will auction an additional $75 billion in super-safe Treasury securities to big investment firms, part of an ongoing effort to help strained credit markets.

The auction — the fifth of its kind — will be held Thursday.

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 investment firms.

The auction program is intended to help financial institutions and the troubled mortgage market.

The goal is to make investment houses more inclined to lend to each other. It also is aimed at providing relief to the distressed market for mortgage-linked securities. Questions about their value and dumping of these securities had driven up mortgage rates, aggravating the housing slump.

The lending program is one of several unconventional steps the Fed has taken to deal with a credit crisis.

Credit troubles worsened earlier this year, driving investment firm Bear Stearns to the brink of bankruptcy and spurring fears other big Wall Street companies could be in jeopardy.

Wanting to avert a broader panic that could endanger the entire U.S. financial system, the Fed agreed last month to temporarily let investment firms obtain emergency loans directly from the Fed, a privilege that only commercial banks had been granted. The decision marked the broadest extension of the Fed's lending authority since the 1930s.