WASHINGTON — Long-term U.S. mortgage rates rose this week, marking new highs for the year. An increase in a key interest rate is expected by the Federal Reserve next week, propelled by signs of strength in the economy.

Mortgage buyer Freddie Mac says the rate on 30-year, fixed-rate loans climbed to 4.21 percent from 4.10 percent last week. The benchmark rate stood at 3.68 percent a year ago and averaged 3.65 percent through 2016, the lowest level in records dating to 1971.

The rate on 15-year mortgages increased to 3.42 percent from 3.32 percent last week.