WASHINGTON — Average long-term U.S. mortgage rates dropped this week to their lowest levels since May.

Mortgage giant Freddie Mac said Thursday the average rate on a 30-year fixed-rate mortgage fell to 3.84 percent from 3.93 percent a week earlier. The benchmark rate hasn’t been that low since May 21.

The rate on 15-year fixed-rate mortgages declined this week to 3.06 percent from 3.15 percent.

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.