WASHINGTON (AP) — Average long-term U.S. mortgage rates rose slightly this week but remained close to historically low levels with the spring home-buying season underway.

Mortgage giant Freddie Mac said Thursday the national average for a 30-year fixed-rate mortgage edged up to 3.67 percent from 3.66 percent last week.

The average rate for a 15-year mortgage, popular with homeowners who refinance, rose to 2.94 percent from 2.93 percent last week.

A year ago, the average 30-year mortgage rate was 4.27 percent and the 15-year rate was 3.33 percent.

The 30-year average rate reached a record low of 3.35 percent in November and December 2012, when the 15-year rate hit a record-low 2.66 percent.

Mortgage rates have remained low even though the Federal Reserve last October ended its monthly bond purchases, designed to hold down long-term rates. The Fed signaled recently that it's still not ready to start raising short-term rates, after keeping them near zero for more than six years.

Government data issued Thursday showed that U.S. homebuilders opened the home-buying season in March at a slower pace than last year, a warning that recent hiring gains have failed to translate into a stronger real estate market.

Construction firms broke ground at a seasonally-adjusted annual rate of 926,000 homes last month, a 2.5 percent decline from the pace in March 2014, the Commerce Department reported. Steady job growth, low mortgage rates and cheaper gasoline have given consumers more flexibility. But the improved economy has yet to significantly boost sales and construction, even as economists say that the gains should soon flow into housing.

To calculate average mortgage rates, Freddie Mac surveys lenders across the country at the beginning of each week. The average doesn't include extra fees, known as points, which most borrowers must pay to get the lowest rates. One point equals 1 percent of the loan amount.

The average fee for a 30-year mortgage rose to 0.7 point from 0.6 point last week. The fee for a 15-year mortgage fell to 0.5 point from 0.6 point.

The average rate on a five-year adjustable-rate mortgage increased to 2.88 percent from 2.83 percent. The fee was unchanged at 0.5 point.

For a one-year ARM, the average rate remained at 2.46 percent. The fee stayed at 0.4 point.