NEW YORK (CNNMoney.com) - Mortgage rates returned to four-year highs, after dipping slightly in the prior week, Freddie Mac said Thursday.

The average rate on 30-year fixed-rate mortgages floated up to 6.60 percent, for the week ending May 11, from the prior week's 6.58 percent. In the year-ago period, the 30-year mortgage rate averaged 5.71 percent.

The average rate hasn't been higher since June 2002, when it averaged 6.63 percent.

"While financial markets try to decipher the spate of recently released economic reports, mortgage rates drifted slightly higher," said Frank Nothaft, Freddie Mac vice president and chief economist. "The current debate is between rising inflation and slower consumer spending. Until the market finds out which influence will be the strongest, mortgage rates should continue to fluctuate as they have the last couple weeks.

The average rate on 15-year fixed-rate mortgages jumped to 6.20 percent from 6.17 percent last week. A year ago, that loan averaged 5.27 percent.

Five-year adjustable-rate mortgages averaged 6.23 percent, up 0.01 from last week. The five-year ARM averaged 5.07 percent last year.

The average one-year adjustable-rate mortgage remained at 5.62 percent. At this time last year, the one-year loan averaged 4.26 percent.

In spite of this week's stability, all rates are up for the year. For homeowners using adjustable rate mortgages (ARMs), a rise in interest rates can mean ballooning payments.

The Mortgage Bankers Association estimates that some $330 billion worth of ARMs will adjust in 2006 and $1 trillion worth will reset by the end of 2007. With a $200,000 loan adjusting upward from 4 percent to 6 percent, the monthly bill would increase to about $1,200, from $955.

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