U.S. 30-year fixed mortgage rates rose for only the second time in the past 14 weeks on modest economic improvements, mortgage financier Freddie Mac said Thursday.

The 30-year rate rose to 3.55 percent in the week ending Aug. 2, up from 3.49 percent in the previous week but far below the 4.39 percent rate during the same period last year. The 15-year mortgage rate rose to 2.83 percent from 2.80 percent in the past week. The five-year Treasury-indexed adjustable rate mortgage (ARM) rate rose to 2.75 percent from 2.74 percent in the past week. The one-year ARM fell slightly to 2.70 percent from 2.71 percent in the prior week.

"Recent announcements of additional debt relief for the euro zone and mixed domestic economic indicators added upward pressure on Treasury yields as well as mortgage rates this week," said Frank Nothaft, Freddie Mac's chief economist, in a statement. GDP growth slowed in the second quarter to a rate of 1.5 percent and pending home sales fell, but consumer confidence rose in July and the Case-Shiller Housing Index rose in May.

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