The number of houses for sale in King County has hit yet another record low, according to statistics released Monday by the Northwest Multiple Listing Service.

“There’s just nothing available out there,” said Glenn Crellin, associate director of research at the University of Washington’s Runstad Center for Real Estate Studies.

Just 2,945 houses were listed for sale as of Dec. 31, the service said, 46 percent fewer than on the same day a year earlier.

Inventory was down 21 percent from November — and that was the first month since at least 1999 in which the number of houses for sale in the county dipped below 4,000, said real-estate blogger Tim Ellis of Seattlebubble.com.

“Now it’s below 3,000,” he said. “That’s just amazing.”

Shrinking inventory may be a factor in rising home prices, the listing service said in releasing its December market report: The median price of houses sold in King County last month was $380,046, up nearly 19 percent from the same month in 2011.

But the dearth of houses for sale also may be turning off some potential buyers, Ellis said.

Pending sales — offers accepted by sellers that haven’t yet closed — were up just 1.6 percent in December year-over-year, the smallest increase since early 2011.

Closed sales, however, rose nearly 19 percent. And they helped dry up an already dry pipeline.

While buyers closed on more than 1,700 houses in King County in December, sellers listed fewer than 1,100 new houses during the month.

Some of that is seasonal: New listings traditionally tail off during the holidays. But December marked an unprecedented fourth straight month in which pending sales outpaced new listings, the listing service said.

Inventory — or lack of it — has been driving the Seattle area real-estate market for several months. Brokers and analysts attribute it mostly to the large number of “underwater” homeowners who are disinclined to sell because they owe lenders more than their houses are worth.

The UW’s Crellin predicted inventory will increase in coming months, in part because rising home prices mean fewer homeowners are underwater.

But Ellis said the number of new listings during the first week of the new year isn’t promising.

Crellin attributed the 19 percent year-over-year increase in the median sale price mostly to a change in the mix of houses that changed hands. “More higher-priced houses are selling now,” he said.

Sales volume was up 33 percent on the Eastside, the county’s priciest area, but fell more than 5 percent in Southwest King County, the least-expensive submarket.

Also, lower-priced bank-repossessed houses make up a smaller percentage of sales now than a year ago.

After hitting post-bubble lows last winter, the median price has hovered between $370,000 and $385,000 since June.

King County closed condo sales were up nearly 17 percent year-over-year in December, but pending sales fell 6 percent.

The median price, $206,000, was more than 11 percent higher than in the same month last year.

In Snohomish County, single-family closings were up just 2 percent from December 2011, while pending sales dropped 13 percent. The median closing price rose 13 percent, to $275,607.

Eric Pryne: epryne@seattletimes.com or 206-464-2231