WASHINGTON (AP) – June 5, 2015 – Average long-term U.S. mortgage rates were flat to slightly lower this week, remaining near their highest levels so far this year.

Mortgage giant Freddie Mac said Thursday the average rate on a 30-year fixed-rate mortgage was unchanged from a week earlier at 3.87 percent. The rate on 15-year fixed-rate mortgages slipped to 3.08 percent from 3.11 percent.

Despite their climb in recent weeks during the height of the spring homebuying season, mortgage rates remain low by historic standards. A year ago, the average 30-year rate was 4.14 percent and the 15-year was 3.23 percent.

Mortgage rates have risen in recent weeks amid signs of improvement in the economy. The economic strength has sparked speculation among investors that the Federal Reserve could move soon to raise short-term interest rates, after keeping them near zero for more than six years.

As a result, bond prices have fallen, pushing bond yields higher. The yield on the 10-year Treasury note climbed to 2.33 percent Wednesday from 2.13 percent a week earlier. The 10-year note traded at 2.31 percent Thursday morning.

Mortgage rates often follow the yield on the 10-year note, so they could trend higher.

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 was unchanged from last week at 0.6 point. The fee for a 15-year loan remained at 0.5 point.

The average rate on five-year adjustable-rate mortgages rose to 2.96 percent from 2.90 percent; the fee remained at 0.5 point. The average rate on one-year ARMs jumped to 2.59 percent from 2.50 percent; the fee slipped to 0.2 point from 0.3 point.