WASHINGTON — Average U.S. rates on fixed mortgages edged up slightly this week, remaining near historically low levels.Mortgage buyer Freddie Mac said Thursday that the nationwide average rate for a 30-year loan rose to 4.15 percent from 4.12 percent last week. The average for the 15-year mortgage increased to 3.24 percent from 3.22 percent.Mortgage rates are slightly lower than they were at the same time last year, having fallen recently after climbing last summer. That's when the Federal Reserve began talking about reducing the monthly bond purchases it has been using to keep long-term interest rates low.Rates have fallen modestly this year as Fed officials have signaled strongly that while they are trimming the bond purchases, they are in no rush to start boosting a key short-term rate the Fed controls.To calculate average mortgage rate, Freddie Mac surveys lenders across the country between Monday and Wednesday 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 this week from 0.5 point last week. The fee for a 15-year loan, popular in refinancing, increased to 0.6 point from 0.5 point last week.The average rate on a one-year adjustable-rate mortgage rose to 2.40 percent from 2.38 percent. The average fee was unchanged at 0.4 point.The average rate on a five-year adjustable-rate mortgage ticked up to 2.99 percent from 2.98 percent. The fee remained at 0.4 point.