WASHINGTON (AP) — Average long-term U.S. mortgage rates rose slightly this week, but remain near historic lows with the spring home-buying season underway and showing strong sales.

Mortgage giant Freddie Mac says the national average for a 30-year fixed -rate mortgage increased to 3.68 percent from 3.65 percent last week.

The average rate for a 15-year mortgage, popular with homeowners who refinance, ticked up to 2.94 percent from 2.92 percent.

A year ago, the 30-year mortgage rate averaged 4.29 percent while the 15-year mortgage averaged 3.38 percent.

Rates have remained low despite the Federal Reserve's decision last year to stop monthly bond purchases meant to hold down long-term rates. The Fed downgraded its view of the economy on Wednesday and offered no sign that a rate increase might be coming soon.