WASHINGTON - Nearly 10 million Americans remain financially trapped by homes worth less than their mortgage debts - an enduring drag on the U.S. economy almost seven years after the housing bust triggered the Great Recession.

During the first three months of this year, 18.8 percent of homeowners with a mortgage - 9.7 million - owed more on their loans than their properties would sell for, according to online real estate database Zillow. Though that was an improvement from the 25.4 percent figure of a year ago, the share of such "underwater" homeowners is about four times the historic average.

An additional 18.1 percent of mortgage holders were "effectively" underwater: They had equity, but the proceeds from selling their home would be too low to recoup the sales costs and put a downpayment on a new property.

The consequence is that few Americans are putting their homes on the market, thereby limiting the economic growth made possible by sales. Because of the shortage of homes being listed, bidding wars have inflated prices in parts of the country to levels that squeeze out many first-time and middle-class buyers.

The problem is most pronounced among starter homes with prices averaging around $100,000, 30.2 percent of whose owners are burdened by underwater mortgages, sometimes called negative equity.

"The unfortunate reality is that housing markets look to be swimming with underwater borrowers for years to come," said Stan Humphries, chief economist at Zillow.

The share of mortgage holders with negative equity is projected to drop to 17 percent at the start of next year, according to Zillow.

Several major U.S. metro areas are stuck with residents who have high rates of negative equity. In Chicago, almost 45 percent are underwater or effectively underwater. The rate is 53.1 percent in Atlanta, 50.6 percent in Las Vegas, 46.6 percent in Charlotte, 44 percent in St. Louis, and 43.2 percent in Tampa.

Figures for Charleston weren't included in the Zillow report.

CoreLogic real estate information firm said in March that 12,952 local residences with a mortgage, or about 9 percent of the total, were classified as underwater for the fourth quarter last year. That was down from 14,473, or almost 10 percent, from the previous three months.

The firm also found that another 5,608 Charleston-area residences, or 3.8 percent, were nearing the negative equity point in the last quarter of 2013. That was down from 4.4 percent, or 6,431 homes, in the third quarter.

The Post and Courier contributed to this report.