U.S. fixed mortgage rates this week increased from the previous week's record lows as the housing market continued to improve, said the Primary Mortgage Market Survey released on Thursday by Freddie Mac.

The mortgage giant said that 30-year fixed-rate mortgage (FRM) was 3.95 percent for the week ending Feb. 23, up from last week's 3.87 percent. Last year at this time, the 30-year FRM averaged 4. 95 percent.

The 15-year FRM averaged 3.19 percent, up from its previous week average of 3.16 percent. A year ago at this time, the 15-year FRM averaged 4.22 percent.

The 5-year Treasury-indexed hybrid adjustable-rate mortgage ( ARM) averaged 2.80 percent, down from last week when it averaged 2. 82 percent, and the 1-year Treasury-indexed ARM averaged 2.73 percent this week, down from last week's 2.84 percent.

The average rates don't include extra fees, known as points, which add the rates up by 0.6 percent to 0.8 percent.

Mortgage rates remained below 4 percent because they followed the yield on the 10-year U.S. Treasury note, which stayed around 2 percent.

Frank Nothaft, Freddie Mac's chief economist, said "loans that were seriously delinquent (90 days or more past due plus the foreclosure inventory) fell to 5.3 percent of prime mortgages at the end of 2011, representing the lowest quarterly share since the start of 2009".

To calculate average mortgage rates, Freddie Mac collects rates from lenders across the country on Monday through Wednesday of each week.

Freddie Mac was established by the Congress in 1970 to provide liquidity, stability and affordability to the U.S. residential mortgage markets.