WASHINGTON — Interest rates on 30-year mortgages rose for a third straight week, hitting the highest level in more than four years as investors continued to express worries about inflation.

Freddie Mac, the mortgage company, reported yesterday that rates on 30-year fixed-rate mortgages increased to a nationwide average of 6.78 percent this week, up from 6.71 last week.

It was the highest level for 30-year mortgages since they averaged 6.81 percent the week of May 24, 2002.

While the housing sector has enjoyed five boom years powered by the lowest mortgage rates in four decades, housing sales are expected to decline by 7 percent or more this year as higher mortgage rates make home ownership more costly.

"Financial markets continue to expect more rate hikes by the Fed over the next six months, which has added upward pressure on mortgage rates," said Frank Nothaft, Freddie Mac's chief economist.

"With higher interest rates, the housing market has begun a gradual and orderly reversion towards historical norms," Nothaft said. "New construction, home sales and house price appreciation have all been slowing over the past few months."

Rates have been climbing since Federal Reserve chairman Ben Bernanke expressed concerns earlier this month that inflation was rising at "unwelcome" levels, raising worries about how many more rate hikes there will be before the Fed decides it has done enough to combat inflation.

A variety of mortgages experienced rate increases this week, according to the Freddie Mac survey.

Rates on 15-year fixed-rate mortgages, a popular choice for refinancing, increased to 6.43 percent, up from 6.36 percent last week.

Rates on one-year adjustable rate mortgages rose to 5.82 percent, up from 5.75 last week and the highest level in more than five years.

Rates on five-year adjustable-rate mortgages climbed to 6.39 percent, up from 6.32 last week.

The mortgage rates do not include add-on fees known as points. The one-year ARM carried a nationwide average fee of 0.8 point. The other three mortgage categories carried average fees of 0.5 point.

A year ago, 30-year mortgages averaged 5.62 percent, 15-year mortgages stood at 5.20 percent, one-year ARMs were at 4.33 percent and five-year ARMs averaged 5.19 percent.