WASHINGTON — Rates on 30-year mortgages plunged this week to the lowest level since January, and may fall further after the Federal Reserve launched a new effort to prop up the flailing housing market.

Mortgage finance giant Freddie Mac said yesterday that average rates on 30-year fixed-rate mortgages dropped to 4.98 percent this week, down from 5.03 percent last week.

It was the lowest since the week of Jan. 15, when it was at 4.96 percent, the lowest point in the history of Freddie Mac's survey, which goes back to 1971.

The rate quotes included in Freddie Mac's survey were taken before the Fed said Wednesday it will pump $1.2 trillion into the economy in an effort to lower rates on mortgages.

That could drive mortgage rates down even further, perhaps past record lows.

However, some mortgage brokers were disappointed yesterday, saying lenders had not pushed down rates as dramatically as had been hoped.

Credit remains tight, and lenders that are not connected to banks and must rely on short-term funding are having trouble raising cash.

"The problem is: We're still not seeing the injection of capital from the private sector," said Douglas Braden, a broker with Northern Colorado Mortgage Co.

Plus many lenders, after laying off workers in droves, don't have the capacity to keep up with a refinancing boom.

Interest rates have drifted lower since November when the Federal Reserve pledged to buy up mortgage-backed securities in an effort to bolster the long-suffering housing market.

The Fed expanded that effort Wednesday, announcing plans to buy up to $300 billion of long-term government bonds and $750 billion in additional mortgage-backed securities guaranteed by Fannie Mae and Freddie Mac.