NEW YORK, Dec 31 - U.S. mortgage rates rose in the latest week for a fourth straight week and hit the highest level since August, a closely watched mortgage survey showed on Thursday.

Interest rates on U.S. 30-year fixed-rate mortgages, the most widely used loan, averaged 5.14 percent for the week ended Dec. 31, the highest since the week ending Aug. 27 and up from the previous week's 5.05 percent, according to a survey released by Freddie Mac (FRE.P) (FRE.N), the second-largest U.S. mortgage finance company.

Although long-term mortgage rates rose for the fourth week in a row, they still remain affordable by historical standards, Frank Nothaft, Freddie Mac vice president and chief economist, said in a statement.

Based on today's median loan amount of $138,000, monthly principal and interest payments for a 30-year fixed-rate mortgage are close to one-third less than a decade ago when rates peaked at 8.6 percent in May 2000, Nothaft said .

This translates into almost 50 percent less in interest payments over the full 30-year term, he said.

The rate of the latest week tops that of the year-ago period when the 30-year mortgage rates averaged 5.10 percent.

The 30-year rate had fallen to 4.71 percent four weeks ago, the lowest since Freddie Mac started the survey in 1971.

Mortgage rates are linked to yields on Treasuries and yields on mortgage-backed securities.

Freddie Mac said the 15-year fixed-rate mortgage averaged 4.54 percent in the latest week, up from 4.45 percent the prior week. For rates table, double-click on [ID:nWALVNE51D]

The Mortgage Bankers Association last week said U.S. mortgage applications fell in its most recent survey. For details, double-click on [ID:nNYS007641]

One-year adjustable-rate mortgages (ARMs) were 4.33 percent in the latest week, down from 4.38 percent the prior week. The rate on the 5/1 ARM, set at a fixed rate for five years and adjustable each following year, was 4.44 percent, compared with 4.40 percent a week earlier.

A year ago, 15-year mortgages averaged 4.83 percent, the one-year ARM 4.85 percent and the 5/1 ARM 5.57 percent. (Editing by Leslie Adler)