The increase in listings from a survey of Multiple Listing Services can be attributed to the extension and expansion of U.S. home buyer tax credits, which is prompting sellers to list their homes sooner rather than wait for spring, the peak home buying season, according to Emeryville, California-based ZipRealty.

It may also be indicative of banks starting to put foreclosed properties up for sale.

The total number of single-family homes and condos listed for sale increased in January from December by 2.9 percent, bringing the total number of active listings in the 27 major U.S. metropolitan markets to 567,265, the company said.

The rise means more than 15,000 additional homes were listed for sale in January in these markets.

Increased supply could negatively impact the hard-hit U.S. housing market, which remains highly vulnerable to setbacks.

More inventory on the market may hurt what is still a huge imbalance between supply and demand. Stabilization of the housing market is seen as key to an economic recovery in the United States.

The number of home listings year-over-year, however, was down 22.33 percent, with 163,000 fewer homes on the market, it said.

"Serious sellers need to list their home now rather than wait for the spring to capitalize on buyers looking to take advantage of the tax credit extension," Patrick Lashinsky, ZipRealty president and CEO, said in a statement.

"While the number of homes for sale is starting to increase, we are still seeing some markets with a shortage of homes for sale," he said.

The first-time home buyer tax credit was originally set to expire November 30.

In November the Obama administration extended the $8,000 first-time buyer credit, added a $6,500 provision for move-up buyers and increased income limits. The eligible borrowers must sign contracts by April 30 and close loans by June 30, 2010.

The average median list price of $258,634 was relatively flat in January, down 1 percent, or $2,521, the company said.

ZipRealty's Housing Inventory Index for January, compiled from local MLS data, showed that inventory increased in most markets. But some markets experienced a drop in inventory in January compared to December.