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 record of 4.78 percent was set during the week ending April 30 and 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 still 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 drop reflects a rush of investors into the security of government debt after concerns about financial trouble in Dubai drove investors to safe harbors. But rates climbed back 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.

And millions of American families have not been able to take advantage of them, particularly in the areas where home prices have fallen the most.

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

That makes refinancing difficult.

While the government has launched a program designed to help these "underwater" borrowers, only about 140,000 homeowners have used it so far.

In Orlando, mortgage broker Chris Brown says the low rates are a boon to first-time homebuyers who can qualify for a loan. But he says he isn't getting much business from homeowners looking to refinance.

"Most of the people that could refinance probably have" done so, he said. "Rates have been artificially low for quite some time."

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

Rates on five-year, adjustable-rate mortgages averaged 4.19 percent, up from 4.18 percent a week earlier. Rates on one-year, adjustable-rate mortgages fell to 4.25 percent from 4.35 percent.