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The percentage of homeowners with mortgages who owe more than their homes are worth continued to rise during the first quarter, but only 1 in 10 of underwater homeowners are seriously delinquent on their loans, according to estimates released today by real estate search portal Zillow.

Zillow—which looks at outstanding loan amounts on individual properties, and compares them with estimated valuations for each home that are generated by computer models—estimates that 15.7 million homes were underwater during the first three months of the year.

That's 31.4% of all homes with mortgages, up from 31.1% during the last three months of 2011 (not all homeowners have mortgages).

Although just 10.1% of underwater homeowners were more than 90 days behind on their mortgage payments, that represents nearly 1.6 million homes that could eventually hit the market as distressed properties.

Numbers like that can put fear into the hearts of would-be home buyers, since distressed properties sell at discounts that can drag down home prices. Those effects are highly dependent on individual market conditions.

Zillow estimates that nationwide, about 2.4 million underwater homeowners owe more than double what their home is worth. In the Las Vegas metro area, 26.8% of homeowners with mortgages are that deeply underwater—nearly 90,000 homes.

At the state level, Nevada had the highest percentage of negative equity—66.9% of homeowners with mortgages were underwater—followed by Arizona (52.3%), Georgia (46.8%), Florida (46.3%) and Michigan (41.7%).

Negative equity "remains an issue for the housing market as a whole, and poses a risk to any recovery," Zillow Chief Economist Stan Humphries said in a statement. "Not only does negative equity tie many to their homes, by making homeowners unable to move when they may want to, but if economic growth slows and unemployment rises, more homeowners will be unable to make timely mortgage payments, increasing delinquency rates and eventually foreclosures."

On the other hand, nearly 40% of underwater homeowners were not deeply underwater, owing between 1% and 20% more than their home is worth.

Humphries said it's important to note that negative equity "remains only a paper loss for the vast majority of underwater homeowners. As home values slowly increase and these homeowners continue to pay down their principal, they will surface again."

The latest numbers from the Mortgage Bankers Association show foreclosure starts and new loan delinquencies fell during the first quarter to their lowest levels since 2007.

The MBA estimates that the delinquency rate for loans on one- to four-unit residential properties decreased to a seasonally adjusted rate of 7.4% of all loans outstanding at the end of the first quarter of 2012, down from 7.58% at the end of 2011 and 8.32% a year ago.

Loans originated at the peak of the housing boom continue to comprise the majority of problem loans, the group said—60% of all loans that were three payments or more past due or in foreclosure were originated between 2005 and 2007.

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Learn more about markets featured in this article: Atlanta, GA.