WASHINGTON — Rates on 30-year mortgages rose for a second straight week, a sharp rebound after hitting a four-month low.

Freddie Mac, the mortgage company, reported yesterday that 30-year fixed-rate mortgages averaged 6.42 percent this week, up from 6.34 percent last week.

Two weeks ago, the nationwide average for 30-year mortgages had dipped to 6.31 percent, the lowest level since May 17. That decline reflected a flight to safety after the turbulence in credit markets, which had increased demand for Treasury securities. Those securities heavily influence mortgage rates.

Rates on 15-year fixed-rate mortgages, a popular choice for refinancing, averaged 6.09 percent this week, up from 5.98 percent last week.

While rates on 30-year and 15-year mortgages rose this week, rates on five-year adjustable-rate mortgages and one-year ARMs declined for a fourth straight week.

The Federal Reserve, which influences short-term rates, announced last week that it was cutting a key rate by a more-than-expected half-point in an effort to make sure that a steep slump in housing and financial market turbulence don't push the economy into a recession.

The Commerce Department reported yesterday that sales of new homes fell by 8.3 percent in August to a seasonally adjusted annual rate of 795,000 units, the slowest level in seven years.

Sales of existing homes were also down in August, dropping 4.3 percent to an annual rate of 5.5 million units, the slowest pace in five years. It marked the sixth straight month that sales of existing homes have declined as the housing market continues to suffer through its steepest downturn in 16 years.