NEW YORK (CNNMoney.com) - The number of people applying for mortgages fell last week, led by a sharp drop in refinancing applications, an industry group said Wednesday.

The Mortgage Bankers Association said its seasonally adjusted index of mortgage application activity for the week ended May 5 fell 5.8 percent to 562.1 from the previous week's 596.8.

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Borrowing costs on 30-year fixed-rate mortgages, excluding fees, averaged 6.61 percent, up 0.04 of a percentage point from the previous week.

The MBA's seasonally adjusted purchase mortgage index fell 3.9 percent to 416.5 from the previous week's 433.3. The index is considered a reliable gauge of U.S. home sales.

The trade group's seasonally adjusted index of refinancing applications fell 8.8 percent to 1427.4 from 1565.6 in the previous week.

The refinance share of mortgage activity decreased to 33.8 percent of total applications, from 35.2 percent last week.

The percentage of adjustable-rate mortgage activity increased to 28.5 percent of applications, from 28.3 percent prior.

Low mortgage rates have been behind a major U.S. housing boom during the last five years.

Fixed 15-year mortgage rates increased to 6.20 percent from 6.19. Rates on one-year adjustable-rate mortgages decreased to 6.04 percent from 6.08 percent.

The MBA's survey covers about 50 percent of residential mortgage bankers and thrifts.

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