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The downturn in home prices has left about 20% of U.S. homeowners owing more on a mortgage than their homes are worth, according to one new study, signaling additional challenges to the Obama administration's efforts to stabilize the housing market.

The increase in the number of such "underwater" borrowers comes amid signs that falling prices are making homes more affordable for first-time buyers and others who have been shut out of the housing market. But falling prices also make it more difficult for homeowners who get into financial trouble to refinance or sell their homes, and for others to take advantage of lower interest rates.

For instance, fewer will qualify to take advantage of a key component of the Obama administration's plan to stabilize the housing market. Under the plan, announced in February, as many as five million homeowners whose loans are owned or guaranteed by government-controlled mortgage giants Fannie Mae and Freddie Mac can refinance their mortgages, but only if the mortgage loan is a maximum of 105% of the home's value.

Government officials are considering an increase in that limit. "It's a question that we're looking at," said James Lockhart, director of the Federal Housing Finance Agency, which regulates Fannie and Freddie.