NEW YORK (CNN/Money) - Mortgage rates surged for the third consecutive week, and lending officials warned that the higher cost of borrowing money could crimp the demand for refinancings, which has been a strong point for a struggling U.S. economy.

The 30-year mortgage rate jumped to 5.94 percent in the week ending July 25, from 5.67 percent a week earlier, with an average of 0.4 of a point payable up front, mortgage lender Freddie Mac reported Thursday. The 30-year averaged 6.34 percent a year ago.

The 15-year fixed-rate mortgage jumped to 5.27 percent, with 0.5 of a point up front, up from 5.00 percent last week but well below the 5.76 percent level of a year ago.

The rate on one-year adjustable-rate mortgages (ARMs), loosely indexed to the 10-year Treasury note, rose to 3.67 percent, with 0.5 point up front, from 3.58 percent last week. At the same time last year, the one-year ARM averaged 4.31 percent.

"Mortgage rates rose again for the third consecutive week, bringing long-term rates to about the same levels we saw at the start of the year," said Frank Nothaft, Freddie Mac's chief economist. "This may start to apply the brakes to the frenzy of refinancing that we are currently experiencing."

Freddie Mac's average mortgage rates are based on a survey of 125 lenders nationwide. The rates include those on mortgages accepted by borrowers with good credit ratings who place a 20 percent down payment on their homes, according to Freddie Mac. The total amount of each mortgage considered for the survey doesn't exceed a $322,700 limit.

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Freddie Mac (FRE: down $1.00 to $50.69, Research, Estimates), or Federal Home Loan Mortgage Corp., is a publicly traded company the government established in 1970 to provide a flow of funds to mortgage lenders. It buys mortgages from banks, bundles them and then resells them as mortgage-backed securities.

Its products, and the products of other similar entities, have become increasingly popular as an alternative to government-backed bonds, particularly with international investors.