WASHINGTON (AP) — The average U.S. rate on a 30-year fixed mortgage rose above 4 percent for the first time in five months. The sharp increase suggests the window to buy or refinance a home at historically low rates is closing.

Mortgage buyer Freddie Mac said Thursday that the rate on the 30-year loan jumped to 4.08 percent, up from 3.92 percent the previous week. A month ago, it touched 3.87 percent, the lowest since long-term mortgages began in the 1950s.

The average on the 15-year fixed mortgage rose to 3.30 percent, up from 3.16 percent last week and a record low of 3.13 percent two weeks ago.

Mortgage rates are rising because they tend to track the yield on the 10-year Treasury note. The economic outlook has improved in recent weeks, leading investors to shift money out of long-term U.S. Treasury bonds and into stocks. That has driven Treasury yields higher.

"With the economy getting stronger, the markets are beginning to recognize that rates are too low," said Joel Naroff, president of Naroff Economic Advisors. "That means mortgage rates should rise."

The average rate on the 30-year mortgage had been at or below 4 percent since last October.

Higher mortgage rates could spur more sales, especially if home prices begin to rise. Potential buyers will likely move quickly to avoid paying higher rates down the line.

"As we move through the year, buyer reaction could be very strong," Naroff said.

The lowest mortgage rates on record have helped lift the housing market in recent months.

January and February made up the best winter for sales of previously occupied homes in five years, when the housing crisis began.

Builders have grown more confident over the past six months after seeing more people express interest in buying a home. They have responded by requesting the most permits to build single-family homes and apartments since October 2008.

Optimism is also rising because the job market has strengthened. Employers have added an average 244,600 jobs per month from December through February. That has helped lower the unemployment rate to 8.3 percent, the lowest level in nearly three years.

Even with the improvement, the housing market is still weak. Millions of foreclosures and short sales — when a lender accepts less than what is owed on a mortgage — remain on the market. And the housing crisis and recession have also persuaded many Americans to rent instead of buy, which has led to a drop in homeownership.

Economists say housing is years away from returning to full health.

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

The average rates don't include extra fees, known as points, which most borrowers must pay to get the lowest rates. One point equals 1 percent of the loan amount.

The average fees for the 30-year and 15-year fixed loans were 0.8, unchanged from 0.8 last week.

For the five-year adjustable loan, the average rate rose to 2.96 percent from 2.83 percent, and the average fee edged down to 0.7 from 0.8.

The average on the one-year adjustable loan rose to 2.84 percent from 2.79 percent, and the average fee was unchanged at 0.6.