WASHINGTON — Average long-term U.S. mortgage rates fell this week, remaining near historically low levels at the start of the spring home-buying season.

The national average for a 30-year fixed-rate mortgage declined to 3.78 percent from 3.86 percent last week. The average rate for a 15-year mortgage slipped to 3.06 percent from 3.10 percent.

A year ago, the average 30-year mortgage stood at 4.32 percent and the 15-year mortgage at 3.32 percent.