Property prices in the States rose last year by 8.3%, the fastest pace in a decade, to a median level of $184,100, government figures show.

But in some big metropolitan areas including Washington and San Francisco, prices are up about 20% and million-dollar homes are becoming increasingly common.

The National Association of Realtors found 23% of homes sold last year were bought by investors, and there are widespread reports of buyers flipping homes for quick short-term gains in many markets.

David Berson, the chief economist for mortgage finance group Fannie Mae, notes that the level of investor ownership of housing hasnt been this high since the late 1980s, which led to a crash in housing prices.

"Many analysts think that a high investor share in the Northeast and California helped exacerbate the housing downturn that happened during the 1990-1991 recession," he wrote in a recent commentary, adding that "the risk of regional home price declines is higher" as so many purchases are by speculators rather than residents.