A five-bedroom house in Las Vegas sold in mid-July for $499,000, double the price it went for three months ago. In Phoenix, a similar house sold this month for $600,000, gaining $273,000 since March.

Bubbles are inflating in Nevada and Arizona even as housing in the rest of the country recovers at a more sustainable pace. Gains in the two desert cities are the biggest since the height of the real estate boom, just before their plunge to the bottom of the national housing collapse. This year, Las Vegas and Phoenix have topped the nation in price increases, according to the S&P/Case-Shiller property-value index.

"They're clearly in bubbles," said Karl Case, one of the creators of the index. "What can go up can go down — real quick."

In May, Phoenix prices jumped 21 percent and in Las Vegas, they rose 23 percent from a year earlier. Nationally, home prices were up 12 percent from a year ago, the most since the beginning of 2006, according to the S&P/Case-Shiller index of 20 cities. Price gyrations in Phoenix and Las Vegas aren't likely to spread and probably don't signal another national calamity, said Daren Blomquist, vice president of data firm RealtyTrac.

"The markets where you see wild swings in prices are destination spots, warm places where people want to vacation, like Las Vegas and Phoenix," Blomquist said. "The rest of the country likely will continue to do well, regardless of what happens in those cities."

There have been regional boom-and-bust cycles before, with home prices in Boston falling 17 percent in a more than four- year decline ending in 1992, and California values plunged 27 percent in six years before bottoming in 1996.

The bust that began in mid-2006 cut 62 percent in the value of Las Vegas homes and 56 percent in Phoenix.

Private-equity firms, hedge funds and real estate investment trusts buying distressed houses to rent have helped push up values in cities hard hit by the housing crash, including Phoenix and Las Vegas. They've raised at least $18 billion and bought more than 100,000 properties since 2011.

In Las Vegas, cash deals made up 60 percent of all transactions in June, according to research firm DataQuick Information Systems Inc.

The bust that began in mid-2006 cut 62 percent in the value of Las Vegas homes and 56 percent in Phoenix.

"The bigger they gained, the harder they fell," said David Blitzer, managing director at Standard & Poor's Financial Services.

The Las Vegas property that doubled its value in three months is a 3,600-square-foot house with five bedrooms and a three-car garage, plus a pool and waterfall in the backyard. It sits on a half-acre lot about 12 miles north of the casinos on Las Vegas Boulevard, known as the Strip.

In Arizona, the 2,800-square-foot house that gained 84 percent in three months has four bedrooms, a pool, and an acre of land north of the city. In both examples, the recent sales information is from RealtyTrac and prior prices are from deeds and assessment records. Both properties were bought and resold by investors.

President Barack Obama, who took office in the midst of the financial crisis brought on by the collapse of the housing market, has warned at least four times this month about what he called "artificial bubbles."

"We have to turn the page on the bubble-and-bust mentality that created this mess," he said in his Aug. 10 weekly radio address.

The real estate markets in Las Vegas and Phoenix are being driven by a shortage of homes, for entirely different reasons. In Phoenix, the dearth of properties is caused by the breakneck pace of foreclosure completions. The inventory of homes in foreclosure in Phoenix dropped 64 percent from a year earlier, the fastest pace in the nation, according to CoreLogic.

In Las Vegas, new consumer protection laws have slowed the pace of repossessions.

A Nevada law passed in 2011 following revelations of foreclosure abuses the previous year requires lenders to prove they have the right to foreclose. Assembly Bill 284 threatens criminal penalties for non-compliance. In the months after it passed, foreclosures ground to a near halt, according to RealtyTrac. The ensuing price spike became known as the 284 bubble.

"Shortages push prices up, and once that happens everyone rushes into the market," Blitzer said.

In May Nevada legislators scrambled to reverse a key provision of the law. A new amendment makes it easier to seize houses by stipulating banks can use affidavits based on a review of their internal records. Still, lenders are moving slowly because they are wary of the shifting legal landscape, RealtyTrac's Blomquist said.

"Banks don't like uncertainty," he said. "The changing laws have muddied the water for banks trying to pursue foreclosures."

In Las Vegas more than half of properties with mortgages are underwater, or worth less than the loans against them, according to Zillow Inc., a real estate data firm. In Phoenix, the share is a third. Nationally, about a quarter of mortgages have negative equity.

To sell a home, owners have to be able to pay off the mortgage, whether from the proceeds of the home sale or with cash. Someone who got a $350,000 mortgage in Phoenix or Las Vegas in 2006 probably is now more than $150,000 underwater, despite the surge in prices.

Those stuck owners give builders the opportunity to gear up to meet demand — at a cost. The average price of an acre of land is $400,000 this month, according to Dennis Smith, CEO of Home Builders Research in Las Vegas. The same parcel would have gone for $200,000 in December, he said.

"The builders said last year that there's no way they would pay more than $200,000 an acre for land," he said. "Now, they're saying there's no way they'll pay more than $400,000."

Labor costs also are rising as the building boom gives workers the opportunity to demand higher pay, Smith said. All of that is passed on to customers, he said.

"The higher that land prices and building costs go, the higher prices will be a year from now," Smith said.

Single-family building permits in Nevada rose in May to the highest point since early 2008. In Arizona, permits reached a five-year peak in June, according to data from the Commerce Department.

In the rest of the U.S., banks are stepping up efforts to reclaim homes and the improving economy is trimming the number of newly delinquent loans as the housing recovery passes the halfway point of its second year. Nationally, there were 2.2 million homes for sale in June, up from 1.8 million in January, a sign the U.S. price gains may slow to a sustainable pace.

That's not the case for the desert cities. The market in Las Vegas is booming at a speed that can't be sustained, Smith said.

"I don't use the word bubble because that implies it's going to burst and never come back," he said. "I don't see a crash. I see a slowdown. That's what better happen, or we'll be seeing a bunch of tumbleweeds blowing through town."

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