WASHINGTON (MarketWatch) -- Pending home sales plunged a seasonally adjusted 16% from October to November as a highly popular tax credit for first-time buyers was set to expire on Nov. 30, the National Association of Realtors reported Tuesday.

The report suggests that sales of existing homes will drop off in the next few months.

The pending sales index, which had risen nine months in a row before falling in November, was 15.5% higher than in November 2008. October's increase was revised higher to 3.9% from 3.7% previously reported. Read the full report from the NAR.

The federal tax credit for first-time buyers was ultimately extended through the first half of 2010, and it's expanded to repeat buyers.

"The fact that pending home sales are comfortably above year-ago levels shows the market has gained sufficient momentum on its own," said Lawrence Yun, chief economist for the lobbying group. "We expect another surge in the spring."

To qualify for the expanded credit, buyers must sign a deal before April 30 and close on the sale before June 30. Here are some tips on getting the credit.

"Sales should rebound going forward," agreed Anna Piretti, economist for BNP Paribas. "Nevertheless, this report suggests that the recent strength of housing demand is still far from becoming self-sustaining and that the housing market remains overly dependent on government support."

Yun said he expects about 2.4 million more buyers to take advantage of the subsidy from taxpayers before it expires on June 30, in addition to 2 million who have already taken the tax credit.

The NAR's pending sales index for November fell in all four regions: down 26% in the Northeast and Midwest, down 15% in the South, and down about 3% in the West.

The index tracks sales contracts signed on existing homes, usually about a month or two before the sale closes, at which point it is picked up in the NAR's existing-home sales report. Existing-home sales are up 39% in the past six months, boosted by the first-time buyer subsidy.

In a separate economic report Tuesday, the Commerce Department said orders for new U.S.-made factory goods rose a seasonally adjusted 1.1% in November, ahead of expectations for a 0.8% gain. Most of the gain came from a 7% increase in petroleum, which was due, in part, to 3% higher prices. See full story on November factory orders.