A sold sign sits next to a house under the final stages of construction in Plano, Texas. After three straight weeks of increases, the average long-term mortgage rates declined this week.

WASHINGTON — Average long-term U.S. mortgage rates declined this week after three straight weeks of increases. The decline could be a spur to prospective buyers as the spring home buying season gets started.

Mortgage buyer Freddie Mac said Thursday the average rate on a 30-year, fixed-rate mortgage slipped to 3.71 percent from 3.73 percent last week. The benchmark rate is above the 3.69 percent level it marked a year ago.

The average rate on 15-year fixed-rate mortgages eased to 2.96 percent from 2.99 percent last week.

After the Federal Reserve’s decision last week to keep a key interest rate unchanged in light of global economic pressures, prices of U.S. government bonds have risen sharply. That has pushed down the yields on the bonds, which mortgage rates follow.

The yield on the 10-year Treasury bond stood at 1.87 percent Wednesday, down from 1.91 percent a week earlier.

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.