NEW YORK (CNN/Money) - Rates for long-term mortgages rose slightly this week, as uncertainty in the U.S. stock markets kept movement to a minimum, mortgage lender Freddie Mac said Thursday.

Freddie Mac reported that the 30-year mortgage averaged 6.18 percent in the week ending Sept. 13, with an average of 0.6 of a point payable up front to the lender. The rate rose from an average 6.15 percent last week and remained lower than its 6.86 percent average a year ago.

The 15-year fixed-rate mortgage averaged 5.59 percent, up from 5.56 percent last week and down from 6.39 percent last year. The 15-year mortgage averaged 0.6 of a point payable up front to the lender.

One-year adjustable-rate mortgages, loosely indexed to the 10-year Treasury note, averaged 4.32 percent with an average 0.6 of a point due up front, down from last week's 4.35 percent average. A year ago the one-year ARM averaged 5.62 percent.

"Markets are on edge, trying to decipher the implications of the latest economic and political statements," said Frank Nothaft, Freddie Mac chief economist. "It's that uncertainty that is keeping mortgage rates from moving one way or the other."

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Freddie Mac (FRE: down $0.60 to $62.91, 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.