Rates on 30-year fixed-rate mortgages are at their lowest since Freddie Mac began tracking them in 1971, the mortgage financier said in releasing the results of its weekly primary mortgage market survey.

Frank Nothaft, Freddie Mac vice president and chief economist, attributed the decline in part to the Federal Reserve's statement this week that it stood ready to expand purchases of mortgage-related assets as it cut the federal funds overnight rate to a record low of zero to 0.25 percent.

The 30-year fixed-rate mortgage (FRM) averaged 5.19 percent with an average 0.7 point for the week ending Dec. 18, down from 5.47 percent last week and 6.14 percent a year ago.

The 15-year FRM averaged 4.92 percent with an average 0.7 point, down from 5.2 percent last week and 5.79 percent a year ago. The 15-year FRM has not been lower since April 1, 2004, when it averaged 4.84 percent.

Rates on adjustable-rate mortgages (ARMs) were also down, with the five-year Treasury-indexed hybrid ARM averaging 5.6 percent with an average 0.6 point, down from 5.82 percent last week and 5.9 percent a year ago.

One-year Treasury-indexed ARMs averaged 4.94 percent with an average 0.5 point, down from 5.09 percent last week and 5.51 percent a year ago.

Meanwhile, the Mortgage Bankers Association said applications for the week ending Dec. 12 were up a seasonally adjusted 2.9 percent, driven by a 6.5 percent jump in refinance applications. Applications for purchase loans fell 4.5 percent, while the government purchase index (largely FHA) was unchanged.