WASHINGTON — Rates on 30-year mortgages climbed to the highest level in 16 months last week, marking the fourth straight week they have been above the 6 percent level.

Mortgage giant Freddie Mac said yesterday that the nationwide average for 30-year, fixed-rate mortgages rose this week to 6.31 percent, from 6.15 percent last week. It was the highest level since 30-year mortgages were at 6.32 percent in June 2004.

The nation's housing market has been booming with sales of both new and existing homes expected to set records for the fifth consecutive year. But economists are forecasting that the sales pace will slow next year under the impact of higher mortgage rates.

Rates on 15-year, fixed-rate mortgages, a popular choice for refinancing a home mortgage, averaged 5.85 percent this week, up from 5.69 percent last week.

One-year adjustable rate mortgages rose to 5.09 percent, up from 4.91 percent last week, and pushed the one-year ARM to its highest level in 3 1/2 years.

The nationwide averages for mortgage rates do not include add-on fees known as points. All four categories of mortgages carried a nationwide average fee of 0.5 point last week.