WASHINGTON - The average interest rate for a 30-year mortgage dropped to a record low of 4.71 percent this week, pushed down by an aggressive government campaign to reduce borrowing costs.

The rate, published Thursday by Freddie Mac, is the lowest since the mortgage finance company began tracking the data in 1971. The previous 4.78 percent record was set during the week ending April 30 and was matched last week.

The Federal Reserve is pumping $1.25 trillion into mortgage-backed securities to try to bring down mortgage rates, but that money is set to run out next spring. The goal of the program is to make home buying more affordable and prop up the housing market.

Despite the government support, qualifying for a loan is tough. Lenders have tightened their standards dramatically, so the best rates are available to those with solid credit and a 20 percent down payment.

Freddie Mac collects mortgage rates on Monday through Wednesday of each week from lenders across the country. Rates often fluctuate significantly, even within a given day, often tracking yields on long-term Treasury bonds.

This week's drop reflects a rush of investors to the security of government debt after concerns about financial trouble in Dubai drove them to safe harbors. Rates climbed back up later in the week, and analysts say they are likely to remain volatile.

"There are no guarantees that mortgage rates are going to stay at these low levels," said Greg McBride, senior financial analyst at Bankrate.com.

Millions of American families have not been able to take advantage of the low rates, particularly in areas where home prices have fallen the most.

About 11 million households, or 23 percent of homeowners with a mortgage, owe more on their loans than their homes are worth, according to First American CoreLogic, a real estate information company.

That makes refinancing difficult.

The average rate on a 15-year fixed-rate mortgage fell to a record low this week of 4.27 percent, down from 4.29 percent last week, according to Freddie Mac.