WASHINGTON - Fixed U.S. mortgage rates rose for the sixth straight week, putting the average rate on the 30-year loan just shy of 4 percent.

Mortgage buyer Freddie Mac said Thursday that the rate on the 30-year loan increased to 3.98 percent. That's up from 3.91 percent last week and the highest since April 2012. The average rate was last at 4 percent or higher in March 2012.

The rate on the 15-year loan advanced to 3.10 percent from 3.03 percent. That's also the highest since April 2012.

Concerns that the Federal Reserve will scale back its bond purchases have pushed rates higher. Still, mortgage rates remain low by historical standards.

Cheap mortgages have helped sustain a housing recovery that began last year, encouraging more Americans to buy homes or refinance existing loans.

Mortgage rates are rising because they tend to follow the yield on the 10-year Treasury note. The yield on the 10-year note climbed as high as 2.29 percent this week from a low of 1.63 percent last month. It has since declined to 2.20 percent in early trading Thursday.

The Fed's $85-billion-a-month in bond purchases have pushed down long-term interest rates. As speculation has grown that the Fed will slow those purchases, investors have driven rates up. That has decreased the value of bonds with lower yields.

Fed policymakers hold a two-day meeting next week that will be closely watched for signals that the Fed may soon slow the bond purchases.

To calculate average mortgage rates, Freddie Mac surveys lenders across the country on Monday through 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 30-year mortgages was unchanged at 0.7 point. The fee for 15-year loans also was steady at 0.7 point.

The average rate on a one-year adjustable-rate mortgage held at 2.58 percent. The fee for one-year adjustable-rate loans was unchanged at 0.4 point.

The average rate on a five-year adjustable-rate mortgage rose to 2.79 percent from 2.74 percent. The fee edged up to 0.6 point from 0.5.