McLEAN, Va. – Mortgage rates have dipped below 5 percent again, four weeks before a government program that is helping keep rates low is scheduled to expire.

The average rate on a 30-year fixed rate mortgage was 4.97 percent last week, down from 5.05 percent a week earlier, mortgage finance company Freddie Mac said Thursday.

Rates dropped to a record low of 4.71 percent in December and have hovered around 5 percent since, kept down by a Federal Reserve campaign to spur homebuying.

The central bank’s $1.25 trillion program to buy up mortgage securities is set to expire March 31. But the Fed has held the door open to extending the program if the economy weakens.

Some analysts argue that rates could rise after the Fed’s program ends, hurting both the recovery in housing and the overall economy. Government officials are optimistic that the Fed will be able to end its program without a major disruption.

In a research note Wednesday, Goldman Sachs analyst Sven Jari Stehn said mortgage rates are expected to see a modest increase after the program ends, though “uncertainty remains significant.”

Also Thursday, data showed pending sales of existing homes dropped 7.6 percent in January from December as stormy weather kept prospective East Coast buyers at home and sales tumbled in the West. The report from the National Association of Realtors was the lowest reading since April and a disappointment to economists, who had expected the metric to rise.

The index has declined for two out of the past three months because home shoppers feel less rushed after a deadline for a homebuyer tax credit was extended from Nov. 30 to April 30.

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

Last week, the average rate on a 15-year fixed-rate mortgage was 4.33 percent, down from 4.4 percent the previous week, according to Freddie Mac.

Rates on five-year, adjustable-rate mortgages averaged 4.11 percent, down from 4.16 percent a week earlier. Rates on one-year, adjustable-rate mortgages rose to 4.27 percent from 4.15 percent.

The rates do not include add-on fees known as points. One point is equal to 1 percent of the total loan amount.

The nationwide fee for loans in Freddie Mac’s survey averaged 0.7 of a point for 30-year and 15-year loans and 0.6 of a point for five-year and one-year loans.