One of every five U.S. home owners owed more on their mortgage than their home was worth in the fourth quarter, a trend that poses a serious threat to the U.S. housing market's recovery, real estate website Zillow.com said on Wednesday.

Homeowners with "underwater" mortgages are more prone to defaults and foreclosures.

They typically do not qualify for refinancings and are unable to sell their homes because they would need to cough up cash at closing time to pay off their mortgage.

The percentage of American single-family homes with mortgages in negative equity rose to 21.4 percent in the fourth quarter from 21 percent in the third quarter, according to the Zillow Real Estate Market Reports.

U.S. home values declined again in the fourth quarter, as the Zillow Home Value Index fell 5 percent year-over-year and down 0.5 percent quarter-over-quarter, to $186,200.

It was the 12th consecutive quarter of year-over-year declines, the reports showed.

"The prevalence of markets in or near a double-dip situation shows that we are not yet at the bottom, in terms of home values," Stan Humphries, Zillow chief economist, said in an interview.

One in five, or 29 of the 143 markets tracked by Zillow, had at least five consecutive month-over-month increases in home values during 2009 before values began to flatten or fall again in the second part of the year.

These markets included the Boston, Atlanta and San Diego metropolitan areas.

Zillow said it defines a "double dip" as two periods of sustained declines in home values separated by a brief period of stabilization or recovery.

Zillow forecasts a definitive bottom in home values in the second quarter of 2010, Humphries said.

"It is important to note, however, that the arrival of the bottom does not mean that recovery is around the corner," he said.

Home values in 29 markets, including the Los Angeles and New York metro areas, increased on a month-over-month basis throughout the fourth quarter.

The rate of increase, however, slowed from November to December in 21 of those markets.

Meanwhile, the number of homeowners losing their homes to foreclosure across the country rose to a new high in December, with more than one in every thousand homes being foreclosed, the highest since Zillow began recording national foreclosure data in 2000, the reports showed.

Foreclosure resales remained high, making up 20.3 percent of all U.S. home sales in December.

Foreclosure resales also made up the majority of sales in several metropolitan areas, including Merced, California, at 68.3 percent; Las Vegas, at 64 percent, and Modesto, California, at 62 percent.

Additionally, 28.5 percent of home sales nationwide sold for less than what the seller originally paid.

Home values increased year-over-year in 27 of 143 markets and remained flat in 15.