Online retail sales in the U.S. have picked up speed in December, after a sluggish start to the holiday shopping season, comScore said Sunday.

During the first five days of December, spending totaled US$3.74 billion, up 9 percent compared with the corresponding period last year, comScore said.

Sales topped $800 million on two of those days: Dec. 1, also known as Cyber Monday for its traditionally heavy online sales following Thanksgiving weekend, and Dec. 2.

People, who normally increase their spending after Thanksgiving and especially in December, leading up to Christmas Day, have been encouraged by big discounts and special offers.

The December surge drove the spending total for the 2008 online holiday shopping season to $14.92 billion, about the same spent during the comparable period last year, comScore said.

The company is forecasting that U.S. online retail spending this holiday season -- November and December -- will be flat compared with last year, due to the economic crisis affecting people's jobs, home values and stock investments. Online retail sales went much better prior to the holiday season, growing 9 percent between January and October, compared with 2007.

The most popular online retailer during the first five days of December was eBay with 36.6 million unique visitors, although that is down 9 percent from the comparable period in 2007, comScore said.

Amazon.com came in second with 29.5 million unique visitors, up 10 percent, while Wal-Mart (19.5 million, up 7 percent), Target (15.9 million [m], down 2 percent) and Apple (11.9 million, up 29 percent) rounded out the top five.

Last week, Nielsen Online reported that unique visitors to online retailers went up 10 percent both on this year's Black Friday -- the big sales day after Thanksgiving -- and Cyber Monday, compared with last year.

Ken Cassar, Nielsen Online's vice president of industry insights, said in a blog post Friday he expects holiday online sales to grow modestly, between 2 percent and 4 percent, compared with 2007.

"The next week and a half are going to be important, but the push by retailers between December 15 and 17 is going to be key. Those are the days that will likely be the biggest of the year, despite the fact that there's no catchy name," Cassar said in his post titled "The Early Read on Holiday 2008: It could be worse."