U.S. mortgage refinancing activity declined to its lowest level in more than 17-1/2 years last week as home borrowing costs rose, the Mortgage Bankers Association said on Wednesday.

The Washington-based trade group said its seasonally adjusted index on applications to refinance an existing home fell 5.9 percent to 884.3 in the week ended Sept. 7. This was the weakest reading since December 2000.

Interest rates on 30-year, fixed-rate conforming mortgages, whose loan balances are $453,100 or less, averaged 4.84 percent, up from 4.80 percent the previous week, MBA said.