The number of Americans who owe more on their mortgages than their homes are worth rose at the end of last year, preventing many people from selling their homes in an already weak housing market.

About 11.1 million households, or 23.1 percent of all mortgaged homes, were "underwater" in the October-December quarter, according to report released Tuesday by housing data firm CoreLogic. That's up from 22.5 percent, or 10.8 million households, in the July-September quarter.

The number of negative home equity mortgages had fallen in the previous three quarters. But that was mostly because more homes had fallen into foreclosure.

Underwater mortgages typically rise when home prices fall. Home prices in December hit their lowest point since the housing bust in 11 of 20 major U.S. metro areas. In a healthy housing market, about 5 percent of homeowners are underwater.

Roughly two-thirds of homeowners in Nevada with a mortgage had negative home equity, the worst in the U.S. Arizona, Florida, Michigan and California were next, with up to 50 percent of homeowners with mortgages in those states underwater.

Oklahoma had the smallest percentage of underwater homeowners in the October-December quarter, at 5.8 percent. Only nine states recorded percentages less than 10 percent.

In addition to the more than 11 million households that are underwater, another 2.4 million homeowners are nearing that point.

The slide in home prices began stabilizing last year. But prices are expected to continue falling in many markets due to still-high levels of foreclosure and unemployment.

That means homes purchased at the height of the real estate boom are unlikely to recover lost value for years.

The total amount of negative equity increased to $751 billion nationwide, up from $744 billion in the previous quarter