The number of U.S. homeowners who are underwater on their mortgages decreased modestly in the third quarter, though levels remained high, data analysis company CoreLogic said Tuesday.

The number of properties with so-called negative equity — in which the amount owed on the mortgage exceeds the property's value — was 10.7 million, or 22.1 percent of all residential properties with a mortgage.

That is a slight decrease from 10.9 million, or 22.5 percent, in the second quarter, CoreLogic said.

"Although slightly down, negative equity remains very high and renders many borrowers vulnerable when negative economic shocks occur, such as job loss or illness," Mark Fleming, chief economist at CoreLogic, said in a statement.

An additional 2.4 million borrowers fell into the near-negative equity camp in the third quarter, defined as those less than 5 percent equity.

Hard-hit Nevada had the highest underwater rate with 58.3 percent of mortgages upside down. The top five was rounded out by Arizona, Florida, Michigan and Georgia.

It was the first time Georgia has been in the top five, knocking out California, which had been in the top five since tracking began in 2009.

As the housing market struggles to recover, the large number of underwater homeowners has prompted concerns of more foreclosures to come if borrowers become unable to keep up with their payments or decide to walk away.

The U.S. government recently expanded a refinancing program in a move that could reach up to 1 million borrowers.