U.S. online sales are up 12 percent to $27.5 billion so far this holiday shopping season compared with a year ago, according to research firm comScore.

In the November 1 through December 17 period, four individual days surpassed $900 million, led by Green Monday on December 13 with $954 million and Free Shipping Day, December 17, with $942 million, according to comScore.

Free Shipping Day punctuated an exceptional week in which consumers spent more than $5.5 billion online, representing a 14 percent increase from last year, said comScore Chairman Gian Fulgoni in a statement.

We have likely already witnessed the peak spending day of the year, which means that Cyber Monday should emerge as the season's heaviest online spending day for the first time in history, Fulgoni said.

The research firm said that computer hardware including Apple Inc's iPad and laptop computers drove most of the growth.

(Reporting by Jennifer Saba; Editing by Diane Craft and Matthew Lewis)

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