(Dow Jones) The proportion of U.S. homeowners who owe more on their mortgages than the properties are worth has swelled to about 23%, threatening prospects for a sustained housing recovery.

Nearly 10.7 million households had negative equity in their homes in the third quarter, according to First American CoreLogic, a real-estate information company based in Santa Ana, Calif. The total number of mortgages nationwide is approximately 47.4 million.

These so-called underwater mortgages pose a roadblock to a housing recovery because the properties are more likely to fall into bank foreclosure and get dumped into an already saturated market. Economists from J.P. Morgan Chase & Co. said Monday they didn't expect U.S. home prices to hit bottom until early 2011, citing the prospect of oversupply.

Home prices have fallen so far that 5.3 million U.S. households are tied to mortgages that are at least 20% higher than their home's value, the First American report said. More than 520,000 of these borrowers have received a notice of default, according to First American.

Most U.S. homeowners still have some equity, and nearly 24 million owner-occupied homes don't have any mortgage, according to the Census Bureau.

But negative equity "is an outstanding risk hanging over the mortgage market," said Mark Fleming, chief economist of First American Core Logic. "It lowers homeowners' mobility because they can't sell, even if they want to move to get a new job." Borrowers who owe more than 120% of their home's value, he said, were more likely to default.

Mortgage troubles are not limited to the unemployed. About 588,000 borrowers defaulted on mortgages last year even though they could afford to pay-more than double the number in 2007, according to a study by Experian and consulting firm Oliver Wyman. "The American consumer has had a long-held taboo against walking away from the home, and this crisis seems to be eroding that," the study said.

Just months after showing signs of leveling off, the housing market has thrown off conflicting signals in recent weeks. Jittery home builders and bad weather led to a 10.6% drop in new home starts in October, and applications for home-purchase mortgages have dropped sharply in recent weeks.

These same falling prices have boosted home sales from the depressed levels of last year. The National Association of Realtors reported Monday that sales of previously occupied homes in October jumped 10.1% from September to a seasonally adjusted annual rate of 6.1 million, the highest since February 2007.

The bump in sales was ahead of forecasts, spurred by falling prices, low mortgage rates and a federal tax credits for buyers. Congress recently expanded and extended the tax credits.