NEW YORK (CNNMoney.com) -- U.S. home prices posted record declines in April, extending a painful losing streak for U.S. home prices.

The S&P/Case-Shiller 20-city Home Price Index fell to a record low of 15.3% on a year-over-year basis, and was down 1.4% from March. The 10-city index was down 16.3% year-over-year and 1.6% for the month.

The 20-city index is based on data going back 19 years, while the 10-city index is 21 years old.

There is one sliver of hope. Although every city surveyed posted year-over-year price drops, the month-to-month pace of declines did slow in many cities. And eight metro areas actually posted gains from March to April.

Bright spots

Hard-hit Cleveland was the biggest winner, with prices up 2.9%. Charlotte, N.C. posted a slight gain of 0.2%, up for the second straight month, while Dallas prices were up 1.1% in April, also up for the second month in a row.

"There might be some regional pockets of improvement, but on an annual basis the overall numbers continue to decline," said David Blitzer, Chairman of the Index Committee at Standard & Poor's.

Indeed, there are anecdotal reports that investors have begun to snap up distressed Cleveland properties at very low prices, according to Dean Baker, Co-Director of the Center for Economic and Policy Research, a Washington-based think tank.

"The data suggests that Cleveland has found a bottom," he said, "although it's just one month's data and I wouldn't make too much of it."

Also on Tuesday, the Office of Federal Housing Enterprise Oversight (OFHEO) reported that its monthly house price purchase index was down 4.6% year-over-year in April.

While the closely-watched Case-Shiller index tracks the sale prices of the same homes over the years, OFHEO's index only tracks sales of homes with mortgages insured by Freddie Mac and Fannie Mae. These loans were for $417,000 or less, until Fannie and Freddie's loan limits were raised in early March.

The overall price declines reported by Case-Shiller have been remarkably consistent over the past two years. Prices on the 20-city index have dropped for 21 straight months, since July 2006. The 10-city index has fallen every month since June 2006.

Declines accelerating

What's more, recent drops have been particularly steep. The 20-city index fell 2.2% in March, 2.6% in February and 2.3% in January, and is now it down another 1.4%.

"In the bubble markets, we continue to see very rapid rates of price declines," said Baker. "If anything, it may be accelerating."

Las Vegas prices plunged 26.8% compared with April of 2007, the worst drop among the 20 cities Case-Shiller covers. Prices there fell 2% in April.

Other hard hit cities include Miami (down 26.7% year-over-year and 4.1% in April), Phoenix (25% and 3.4%) and Los Angeles (23.1% and 2.2%).

"Bubble markets are now trapped in a vicious negative cycle," said Mark Zandi, chief economist for Moody's Economy.com, "with foreclosures driving prices down, which leads to more foreclosures."

Foreclosures account for a much larger proportion of sales than they did a year ago, he said, and that pulls down the numbers. "But just because the average home in your market is down 25%," he said, "doesn't mean that your house is down 25%."

Still, plummeting prices could derail some of the foreclosure prevention efforts underway across the nation. As home prices fall, that wipes out home equity, often leaving homeowners underwater, with mortgages worth more than their homes.

Some 10 million homeowners are now underwater, according to Economy.com, and that number will continue to grow as home prices plummet.

Underwater borrowers have higher rates of foreclosure than those with some home equity, since they can't tap their homes for cash in case of an emergency. And some owners are simply walking away from homes that have lost so much value rather than continuing to make expensive payments every month.

The flood of foreclosures may be darkening an already bleak picture, said Zandi, "but the market is very bad right now."